[PATCH v4 0/2] Fix DR6/DR7 initialization

[PATCH v4 0/2] Fix DR6/DR7 initialization

[Xin Li (Intel)]

Sohil reported seeing a split lock warning when running a test that
generates userspace #DB:

  x86/split lock detection: #DB: sigtrap_loop_64/4614 took a bus_lock trap at address: 0x4011ae


We investigated the issue and figured out:

  1) The warning is a false positive.

  2) It is not caused by the test itself.

  3) It occurs even when Bus Lock Detection (BLD) is disabled.

  4) It only happens on the first #DB on a CPU.


And the root cause is, at boot time, Linux zeros DR6.  This leads to
different DR6 values depending on whether the CPU supports BLD:

  1) On CPUs with BLD support, DR6 becomes 0xFFFF07F0 (bit 11, DR6.BLD,
     is cleared).

  2) On CPUs without BLD, DR6 becomes 0xFFFF0FF0.

Since only BLD-induced #DB exceptions clear DR6.BLD and other debug
exceptions leave it unchanged, even if the first #DB is unrelated to
BLD, DR6.BLD is still cleared.  As a result, such a first #DB is
misinterpreted as a BLD #DB, and a false warning is triggerred.


Fix the bug by initializing DR6 by writing its architectural reset
value at boot time.

DR7 suffers from a similar issue, apply the same fix.


This patch set is based on tip/x86/urgent branch as of today.

Link to the previous patch set v3:
https://lore.kernel.org/all/20250618172723.1651465-1-xin@zytor.com/


Change in v4:
*) Cc stable in the DR7 initialization patch for backporting, just in
   case bit 10 of DR7 has become unreserved on new hardware, even
   though clearing it doesn't currently cause any real issues (Dave
   Hansen).


Xin Li (Intel) (2):
  x86/traps: Initialize DR6 by writing its architectural reset value
  x86/traps: Initialize DR7 by writing its architectural reset value

 arch/x86/include/asm/debugreg.h      | 19 ++++++++++++----
 arch/x86/include/asm/kvm_host.h      |  2 +-
 arch/x86/include/uapi/asm/debugreg.h | 21 ++++++++++++++++-
 arch/x86/kernel/cpu/common.c         | 24 ++++++++------------
 arch/x86/kernel/kgdb.c               |  2 +-
 arch/x86/kernel/process_32.c         |  2 +-
 arch/x86/kernel/process_64.c         |  2 +-
 arch/x86/kernel/traps.c              | 34 +++++++++++++++++-----------
 arch/x86/kvm/x86.c                   |  4 ++--
 9 files changed, 72 insertions(+), 38 deletions(-)


base-commit: 2aebf5ee43bf0ed225a09a30cf515d9f2813b759
-- 
2.49.0

[PATCH v4 1/2] x86/traps: Initialize DR6 by writing its architectural reset value

[Xin Li (Intel)]

Initialize DR6 by writing its architectural reset value to avoid
incorrectly zeroing DR6 to clear DR6.BLD at boot time, which leads
to a false bus lock detected warning.

The Intel SDM says:

  1) Certain debug exceptions may clear bits 0-3 of DR6.

  2) BLD induced #DB clears DR6.BLD and any other debug exception
     doesn't modify DR6.BLD.

  3) RTM induced #DB clears DR6.RTM and any other debug exception
     sets DR6.RTM.

  To avoid confusion in identifying debug exceptions, debug handlers
  should set DR6.BLD and DR6.RTM, and clear other DR6 bits before
  returning.

The DR6 architectural reset value 0xFFFF0FF0, already defined as
macro DR6_RESERVED, satisfies these requirements, so just use it to
reinitialize DR6 whenever needed.

Since clear_all_debug_regs() no longer zeros all debug registers,
rename it to initialize_debug_regs() to better reflect its current
behavior.

Since debug_read_clear_dr6() no longer clears DR6, rename it to
debug_read_reset_dr6() to better reflect its current behavior.

Reported-by: Sohil Mehta <sohil.mehta@intel.com>
Link: https://lore.kernel.org/lkml/06e68373-a92b-472e-8fd9-ba548119770c@intel.com/
Fixes: ebb1064e7c2e9 ("x86/traps: Handle #DB for bus lock")
Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Cc: stable@vger.kernel.org
---

Changes in v3:
*) Polish initialize_debug_regs() (PeterZ).
*) Rewrite the comment for DR6_RESERVED definition (Sohil and Sean).
*) Collect TB, RB, AB (PeterZ and Sohil).

Changes in v2:
*) Use debug register index 6 rather than DR_STATUS (PeterZ and Sean).
*) Move this patch the first of the patch set to ease backporting.
---
 arch/x86/include/uapi/asm/debugreg.h | 21 ++++++++++++++++-
 arch/x86/kernel/cpu/common.c         | 24 ++++++++------------
 arch/x86/kernel/traps.c              | 34 +++++++++++++++++-----------
 3 files changed, 51 insertions(+), 28 deletions(-)

diff --git a/arch/x86/include/uapi/asm/debugreg.h b/arch/x86/include/uapi/asm/debugreg.h
index 0007ba077c0c..41da492dfb01 100644
--- a/arch/x86/include/uapi/asm/debugreg.h
+++ b/arch/x86/include/uapi/asm/debugreg.h
@@ -15,7 +15,26 @@
    which debugging register was responsible for the trap.  The other bits
    are either reserved or not of interest to us. */
 
-/* Define reserved bits in DR6 which are always set to 1 */
+/*
+ * Define bits in DR6 which are set to 1 by default.
+ *
+ * This is also the DR6 architectural value following Power-up, Reset or INIT.
+ *
+ * Note, with the introduction of Bus Lock Detection (BLD) and Restricted
+ * Transactional Memory (RTM), the DR6 register has been modified:
+ *
+ * 1) BLD flag (bit 11) is no longer reserved to 1 if the CPU supports
+ *    Bus Lock Detection.  The assertion of a bus lock could clear it.
+ *
+ * 2) RTM flag (bit 16) is no longer reserved to 1 if the CPU supports
+ *    restricted transactional memory.  #DB occurred inside an RTM region
+ *    could clear it.
+ *
+ * Apparently, DR6.BLD and DR6.RTM are active low bits.
+ *
+ * As a result, DR6_RESERVED is an incorrect name now, but it is kept for
+ * compatibility.
+ */
 #define DR6_RESERVED	(0xFFFF0FF0)
 
 #define DR_TRAP0	(0x1)		/* db0 */
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 8feb8fd2957a..0f6c280a94f0 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2243,20 +2243,16 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
 #endif
 #endif
 
-/*
- * Clear all 6 debug registers:
- */
-static void clear_all_debug_regs(void)
+static void initialize_debug_regs(void)
 {
-	int i;
-
-	for (i = 0; i < 8; i++) {
-		/* Ignore db4, db5 */
-		if ((i == 4) || (i == 5))
-			continue;
-
-		set_debugreg(0, i);
-	}
+	/* Control register first -- to make sure everything is disabled. */
+	set_debugreg(0, 7);
+	set_debugreg(DR6_RESERVED, 6);
+	/* dr5 and dr4 don't exist */
+	set_debugreg(0, 3);
+	set_debugreg(0, 2);
+	set_debugreg(0, 1);
+	set_debugreg(0, 0);
 }
 
 #ifdef CONFIG_KGDB
@@ -2417,7 +2413,7 @@ void cpu_init(void)
 
 	load_mm_ldt(&init_mm);
 
-	clear_all_debug_regs();
+	initialize_debug_regs();
 	dbg_restore_debug_regs();
 
 	doublefault_init_cpu_tss();
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index c5c897a86418..36354b470590 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -1022,24 +1022,32 @@ static bool is_sysenter_singlestep(struct pt_regs *regs)
 #endif
 }
 
-static __always_inline unsigned long debug_read_clear_dr6(void)
+static __always_inline unsigned long debug_read_reset_dr6(void)
 {
 	unsigned long dr6;
 
+	get_debugreg(dr6, 6);
+	dr6 ^= DR6_RESERVED; /* Flip to positive polarity */
+
 	/*
 	 * The Intel SDM says:
 	 *
-	 *   Certain debug exceptions may clear bits 0-3. The remaining
-	 *   contents of the DR6 register are never cleared by the
-	 *   processor. To avoid confusion in identifying debug
-	 *   exceptions, debug handlers should clear the register before
-	 *   returning to the interrupted task.
+	 *   Certain debug exceptions may clear bits 0-3 of DR6.
+	 *
+	 *   BLD induced #DB clears DR6.BLD and any other debug
+	 *   exception doesn't modify DR6.BLD.
 	 *
-	 * Keep it simple: clear DR6 immediately.
+	 *   RTM induced #DB clears DR6.RTM and any other debug
+	 *   exception sets DR6.RTM.
+	 *
+	 *   To avoid confusion in identifying debug exceptions,
+	 *   debug handlers should set DR6.BLD and DR6.RTM, and
+	 *   clear other DR6 bits before returning.
+	 *
+	 * Keep it simple: write DR6 with its architectural reset
+	 * value 0xFFFF0FF0, defined as DR6_RESERVED, immediately.
 	 */
-	get_debugreg(dr6, 6);
 	set_debugreg(DR6_RESERVED, 6);
-	dr6 ^= DR6_RESERVED; /* Flip to positive polarity */
 
 	return dr6;
 }
@@ -1239,13 +1247,13 @@ static noinstr void exc_debug_user(struct pt_regs *regs, unsigned long dr6)
 /* IST stack entry */
 DEFINE_IDTENTRY_DEBUG(exc_debug)
 {
-	exc_debug_kernel(regs, debug_read_clear_dr6());
+	exc_debug_kernel(regs, debug_read_reset_dr6());
 }
 
 /* User entry, runs on regular task stack */
 DEFINE_IDTENTRY_DEBUG_USER(exc_debug)
 {
-	exc_debug_user(regs, debug_read_clear_dr6());
+	exc_debug_user(regs, debug_read_reset_dr6());
 }
 
 #ifdef CONFIG_X86_FRED
@@ -1264,7 +1272,7 @@ DEFINE_FREDENTRY_DEBUG(exc_debug)
 {
 	/*
 	 * FRED #DB stores DR6 on the stack in the format which
-	 * debug_read_clear_dr6() returns for the IDT entry points.
+	 * debug_read_reset_dr6() returns for the IDT entry points.
 	 */
 	unsigned long dr6 = fred_event_data(regs);
 
@@ -1279,7 +1287,7 @@ DEFINE_FREDENTRY_DEBUG(exc_debug)
 /* 32 bit does not have separate entry points. */
 DEFINE_IDTENTRY_RAW(exc_debug)
 {
-	unsigned long dr6 = debug_read_clear_dr6();
+	unsigned long dr6 = debug_read_reset_dr6();
 
 	if (user_mode(regs))
 		exc_debug_user(regs, dr6);
-- 
2.49.0

Re: [PATCH v4 1/2] x86/traps: Initialize DR6 by writing its architectural reset value

[Ethan Zhao]

在 2025/6/21 7:15, Xin Li (Intel) 写道:
> Initialize DR6 by writing its architectural reset value to avoid
> incorrectly zeroing DR6 to clear DR6.BLD at boot time, which leads
> to a false bus lock detected warning.
>
> The Intel SDM says:
>
>    1) Certain debug exceptions may clear bits 0-3 of DR6.
>
>    2) BLD induced #DB clears DR6.BLD and any other debug exception
>       doesn't modify DR6.BLD.
>
>    3) RTM induced #DB clears DR6.RTM and any other debug exception
>       sets DR6.RTM.
>
>    To avoid confusion in identifying debug exceptions, debug handlers
>    should set DR6.BLD and DR6.RTM, and clear other DR6 bits before
>    returning.
>
> The DR6 architectural reset value 0xFFFF0FF0, already defined as
> macro DR6_RESERVED, satisfies these requirements, so just use it to
> reinitialize DR6 whenever needed.
>
> Since clear_all_debug_regs() no longer zeros all debug registers,
> rename it to initialize_debug_regs() to better reflect its current
> behavior.
>
> Since debug_read_clear_dr6() no longer clears DR6, rename it to
> debug_read_reset_dr6() to better reflect its current behavior.
>
> Reported-by: Sohil Mehta <sohil.mehta@intel.com>
> Link: https://lore.kernel.org/lkml/06e68373-a92b-472e-8fd9-ba548119770c@intel.com/
> Fixes: ebb1064e7c2e9 ("x86/traps: Handle #DB for bus lock")
> Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
> Tested-by: Sohil Mehta <sohil.mehta@intel.com>
> Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
> Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
> Cc: stable@vger.kernel.org
> ---
>
> Changes in v3:
> *) Polish initialize_debug_regs() (PeterZ).
> *) Rewrite the comment for DR6_RESERVED definition (Sohil and Sean).
> *) Collect TB, RB, AB (PeterZ and Sohil).
>
> Changes in v2:
> *) Use debug register index 6 rather than DR_STATUS (PeterZ and Sean).
> *) Move this patch the first of the patch set to ease backporting.
> ---
>   arch/x86/include/uapi/asm/debugreg.h | 21 ++++++++++++++++-
>   arch/x86/kernel/cpu/common.c         | 24 ++++++++------------
>   arch/x86/kernel/traps.c              | 34 +++++++++++++++++-----------
>   3 files changed, 51 insertions(+), 28 deletions(-)
>
> diff --git a/arch/x86/include/uapi/asm/debugreg.h b/arch/x86/include/uapi/asm/debugreg.h
> index 0007ba077c0c..41da492dfb01 100644
> --- a/arch/x86/include/uapi/asm/debugreg.h
> +++ b/arch/x86/include/uapi/asm/debugreg.h
> @@ -15,7 +15,26 @@
>      which debugging register was responsible for the trap.  The other bits
>      are either reserved or not of interest to us. */
>   
> -/* Define reserved bits in DR6 which are always set to 1 */
> +/*
> + * Define bits in DR6 which are set to 1 by default.
> + *
> + * This is also the DR6 architectural value following Power-up, Reset or INIT.
> + *
> + * Note, with the introduction of Bus Lock Detection (BLD) and Restricted
> + * Transactional Memory (RTM), the DR6 register has been modified:
> + *
> + * 1) BLD flag (bit 11) is no longer reserved to 1 if the CPU supports
> + *    Bus Lock Detection.  The assertion of a bus lock could clear it.
> + *
> + * 2) RTM flag (bit 16) is no longer reserved to 1 if the CPU supports
> + *    restricted transactional memory.  #DB occurred inside an RTM region
> + *    could clear it.
> + *
> + * Apparently, DR6.BLD and DR6.RTM are active low bits.
> + *
> + * As a result, DR6_RESERVED is an incorrect name now, but it is kept for
> + * compatibility.
> + */
>   #define DR6_RESERVED	(0xFFFF0FF0)
>   
>   #define DR_TRAP0	(0x1)		/* db0 */
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 8feb8fd2957a..0f6c280a94f0 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2243,20 +2243,16 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
>   #endif
>   #endif
>   
> -/*
> - * Clear all 6 debug registers:
> - */
> -static void clear_all_debug_regs(void)
> +static void initialize_debug_regs(void)
>   {
> -	int i;
> -
> -	for (i = 0; i < 8; i++) {
> -		/* Ignore db4, db5 */
> -		if ((i == 4) || (i == 5))
> -			continue;
> -
> -		set_debugreg(0, i);
> -	}
> +	/* Control register first -- to make sure everything is disabled. */

In the Figure 19-1. Debug Registers of SDM section 19.2 DEBUG REGISTERS,

bit 10, 12, 14, 15 of DR7 are marked as gray (Reversed) and their value 
are filled as

1, 0, 0,0 ; should we clear them all here ?  I didn't find any other 
description in the

SDM about the result if they are cleaned. of course, this patch doesn't 
change

the behaviour of original DR7 initialization code, no justification needed,

just out of curiosity.


Thanks,

Ethan

> +	set_debugreg(0, 7);
> +	set_debugreg(DR6_RESERVED, 6);
> +	/* dr5 and dr4 don't exist */
> +	set_debugreg(0, 3);
> +	set_debugreg(0, 2);
> +	set_debugreg(0, 1);
> +	set_debugreg(0, 0);
>   }
>   
>   #ifdef CONFIG_KGDB
> @@ -2417,7 +2413,7 @@ void cpu_init(void)
>   
>   	load_mm_ldt(&init_mm);
>   
> -	clear_all_debug_regs();
> +	initialize_debug_regs();
>   	dbg_restore_debug_regs();
>   
>   	doublefault_init_cpu_tss();
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index c5c897a86418..36354b470590 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -1022,24 +1022,32 @@ static bool is_sysenter_singlestep(struct pt_regs *regs)
>   #endif
>   }
>   
> -static __always_inline unsigned long debug_read_clear_dr6(void)
> +static __always_inline unsigned long debug_read_reset_dr6(void)
>   {
>   	unsigned long dr6;
>   
> +	get_debugreg(dr6, 6);
> +	dr6 ^= DR6_RESERVED; /* Flip to positive polarity */
> +
>   	/*
>   	 * The Intel SDM says:
>   	 *
> -	 *   Certain debug exceptions may clear bits 0-3. The remaining
> -	 *   contents of the DR6 register are never cleared by the
> -	 *   processor. To avoid confusion in identifying debug
> -	 *   exceptions, debug handlers should clear the register before
> -	 *   returning to the interrupted task.
> +	 *   Certain debug exceptions may clear bits 0-3 of DR6.
> +	 *
> +	 *   BLD induced #DB clears DR6.BLD and any other debug
> +	 *   exception doesn't modify DR6.BLD.
>   	 *
> -	 * Keep it simple: clear DR6 immediately.
> +	 *   RTM induced #DB clears DR6.RTM and any other debug
> +	 *   exception sets DR6.RTM.
> +	 *
> +	 *   To avoid confusion in identifying debug exceptions,
> +	 *   debug handlers should set DR6.BLD and DR6.RTM, and
> +	 *   clear other DR6 bits before returning.
> +	 *
> +	 * Keep it simple: write DR6 with its architectural reset
> +	 * value 0xFFFF0FF0, defined as DR6_RESERVED, immediately.
>   	 */
> -	get_debugreg(dr6, 6);
>   	set_debugreg(DR6_RESERVED, 6);
> -	dr6 ^= DR6_RESERVED; /* Flip to positive polarity */
>   
>   	return dr6;
>   }
> @@ -1239,13 +1247,13 @@ static noinstr void exc_debug_user(struct pt_regs *regs, unsigned long dr6)
>   /* IST stack entry */
>   DEFINE_IDTENTRY_DEBUG(exc_debug)
>   {
> -	exc_debug_kernel(regs, debug_read_clear_dr6());
> +	exc_debug_kernel(regs, debug_read_reset_dr6());
>   }
>   
>   /* User entry, runs on regular task stack */
>   DEFINE_IDTENTRY_DEBUG_USER(exc_debug)
>   {
> -	exc_debug_user(regs, debug_read_clear_dr6());
> +	exc_debug_user(regs, debug_read_reset_dr6());
>   }
>   
>   #ifdef CONFIG_X86_FRED
> @@ -1264,7 +1272,7 @@ DEFINE_FREDENTRY_DEBUG(exc_debug)
>   {
>   	/*
>   	 * FRED #DB stores DR6 on the stack in the format which
> -	 * debug_read_clear_dr6() returns for the IDT entry points.
> +	 * debug_read_reset_dr6() returns for the IDT entry points.
>   	 */
>   	unsigned long dr6 = fred_event_data(regs);
>   
> @@ -1279,7 +1287,7 @@ DEFINE_FREDENTRY_DEBUG(exc_debug)
>   /* 32 bit does not have separate entry points. */
>   DEFINE_IDTENTRY_RAW(exc_debug)
>   {
> -	unsigned long dr6 = debug_read_clear_dr6();
> +	unsigned long dr6 = debug_read_reset_dr6();
>   
>   	if (user_mode(regs))
>   		exc_debug_user(regs, dr6);

-- 
"firm, enduring, strong, and long-lived"

Re: [PATCH v4 1/2] x86/traps: Initialize DR6 by writing its architectural reset value

[H. Peter Anvin]

On June 22, 2025 11:49:11 PM PDT, Ethan Zhao <haifeng.zhao@linux.intel.com> wrote:
>
>在 2025/6/21 7:15, Xin Li (Intel) 写道:
>> Initialize DR6 by writing its architectural reset value to avoid
>> incorrectly zeroing DR6 to clear DR6.BLD at boot time, which leads
>> to a false bus lock detected warning.
>> 
>> The Intel SDM says:
>> 
>>    1) Certain debug exceptions may clear bits 0-3 of DR6.
>> 
>>    2) BLD induced #DB clears DR6.BLD and any other debug exception
>>       doesn't modify DR6.BLD.
>> 
>>    3) RTM induced #DB clears DR6.RTM and any other debug exception
>>       sets DR6.RTM.
>> 
>>    To avoid confusion in identifying debug exceptions, debug handlers
>>    should set DR6.BLD and DR6.RTM, and clear other DR6 bits before
>>    returning.
>> 
>> The DR6 architectural reset value 0xFFFF0FF0, already defined as
>> macro DR6_RESERVED, satisfies these requirements, so just use it to
>> reinitialize DR6 whenever needed.
>> 
>> Since clear_all_debug_regs() no longer zeros all debug registers,
>> rename it to initialize_debug_regs() to better reflect its current
>> behavior.
>> 
>> Since debug_read_clear_dr6() no longer clears DR6, rename it to
>> debug_read_reset_dr6() to better reflect its current behavior.
>> 
>> Reported-by: Sohil Mehta <sohil.mehta@intel.com>
>> Link: https://lore.kernel.org/lkml/06e68373-a92b-472e-8fd9-ba548119770c@intel.com/
>> Fixes: ebb1064e7c2e9 ("x86/traps: Handle #DB for bus lock")
>> Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>> Tested-by: Sohil Mehta <sohil.mehta@intel.com>
>> Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>> Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
>> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
>> Cc: stable@vger.kernel.org
>> ---
>> 
>> Changes in v3:
>> *) Polish initialize_debug_regs() (PeterZ).
>> *) Rewrite the comment for DR6_RESERVED definition (Sohil and Sean).
>> *) Collect TB, RB, AB (PeterZ and Sohil).
>> 
>> Changes in v2:
>> *) Use debug register index 6 rather than DR_STATUS (PeterZ and Sean).
>> *) Move this patch the first of the patch set to ease backporting.
>> ---
>>   arch/x86/include/uapi/asm/debugreg.h | 21 ++++++++++++++++-
>>   arch/x86/kernel/cpu/common.c         | 24 ++++++++------------
>>   arch/x86/kernel/traps.c              | 34 +++++++++++++++++-----------
>>   3 files changed, 51 insertions(+), 28 deletions(-)
>> 
>> diff --git a/arch/x86/include/uapi/asm/debugreg.h b/arch/x86/include/uapi/asm/debugreg.h
>> index 0007ba077c0c..41da492dfb01 100644
>> --- a/arch/x86/include/uapi/asm/debugreg.h
>> +++ b/arch/x86/include/uapi/asm/debugreg.h
>> @@ -15,7 +15,26 @@
>>      which debugging register was responsible for the trap.  The other bits
>>      are either reserved or not of interest to us. */
>>   -/* Define reserved bits in DR6 which are always set to 1 */
>> +/*
>> + * Define bits in DR6 which are set to 1 by default.
>> + *
>> + * This is also the DR6 architectural value following Power-up, Reset or INIT.
>> + *
>> + * Note, with the introduction of Bus Lock Detection (BLD) and Restricted
>> + * Transactional Memory (RTM), the DR6 register has been modified:
>> + *
>> + * 1) BLD flag (bit 11) is no longer reserved to 1 if the CPU supports
>> + *    Bus Lock Detection.  The assertion of a bus lock could clear it.
>> + *
>> + * 2) RTM flag (bit 16) is no longer reserved to 1 if the CPU supports
>> + *    restricted transactional memory.  #DB occurred inside an RTM region
>> + *    could clear it.
>> + *
>> + * Apparently, DR6.BLD and DR6.RTM are active low bits.
>> + *
>> + * As a result, DR6_RESERVED is an incorrect name now, but it is kept for
>> + * compatibility.
>> + */
>>   #define DR6_RESERVED	(0xFFFF0FF0)
>>     #define DR_TRAP0	(0x1)		/* db0 */
>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>> index 8feb8fd2957a..0f6c280a94f0 100644
>> --- a/arch/x86/kernel/cpu/common.c
>> +++ b/arch/x86/kernel/cpu/common.c
>> @@ -2243,20 +2243,16 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
>>   #endif
>>   #endif
>>   -/*
>> - * Clear all 6 debug registers:
>> - */
>> -static void clear_all_debug_regs(void)
>> +static void initialize_debug_regs(void)
>>   {
>> -	int i;
>> -
>> -	for (i = 0; i < 8; i++) {
>> -		/* Ignore db4, db5 */
>> -		if ((i == 4) || (i == 5))
>> -			continue;
>> -
>> -		set_debugreg(0, i);
>> -	}
>> +	/* Control register first -- to make sure everything is disabled. */
>
>In the Figure 19-1. Debug Registers of SDM section 19.2 DEBUG REGISTERS,
>
>bit 10, 12, 14, 15 of DR7 are marked as gray (Reversed) and their value are filled as
>
>1, 0, 0,0 ; should we clear them all here ?  I didn't find any other description in the
>
>SDM about the result if they are cleaned. of course, this patch doesn't change
>
>the behaviour of original DR7 initialization code, no justification needed,
>
>just out of curiosity.
>
>
>Thanks,
>
>Ethan
>
>> +	set_debugreg(0, 7);
>> +	set_debugreg(DR6_RESERVED, 6);
>> +	/* dr5 and dr4 don't exist */
>> +	set_debugreg(0, 3);
>> +	set_debugreg(0, 2);
>> +	set_debugreg(0, 1);
>> +	set_debugreg(0, 0);
>>   }
>>     #ifdef CONFIG_KGDB
>> @@ -2417,7 +2413,7 @@ void cpu_init(void)
>>     	load_mm_ldt(&init_mm);
>>   -	clear_all_debug_regs();
>> +	initialize_debug_regs();
>>   	dbg_restore_debug_regs();
>>     	doublefault_init_cpu_tss();
>> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
>> index c5c897a86418..36354b470590 100644
>> --- a/arch/x86/kernel/traps.c
>> +++ b/arch/x86/kernel/traps.c
>> @@ -1022,24 +1022,32 @@ static bool is_sysenter_singlestep(struct pt_regs *regs)
>>   #endif
>>   }
>>   -static __always_inline unsigned long debug_read_clear_dr6(void)
>> +static __always_inline unsigned long debug_read_reset_dr6(void)
>>   {
>>   	unsigned long dr6;
>>   +	get_debugreg(dr6, 6);
>> +	dr6 ^= DR6_RESERVED; /* Flip to positive polarity */
>> +
>>   	/*
>>   	 * The Intel SDM says:
>>   	 *
>> -	 *   Certain debug exceptions may clear bits 0-3. The remaining
>> -	 *   contents of the DR6 register are never cleared by the
>> -	 *   processor. To avoid confusion in identifying debug
>> -	 *   exceptions, debug handlers should clear the register before
>> -	 *   returning to the interrupted task.
>> +	 *   Certain debug exceptions may clear bits 0-3 of DR6.
>> +	 *
>> +	 *   BLD induced #DB clears DR6.BLD and any other debug
>> +	 *   exception doesn't modify DR6.BLD.
>>   	 *
>> -	 * Keep it simple: clear DR6 immediately.
>> +	 *   RTM induced #DB clears DR6.RTM and any other debug
>> +	 *   exception sets DR6.RTM.
>> +	 *
>> +	 *   To avoid confusion in identifying debug exceptions,
>> +	 *   debug handlers should set DR6.BLD and DR6.RTM, and
>> +	 *   clear other DR6 bits before returning.
>> +	 *
>> +	 * Keep it simple: write DR6 with its architectural reset
>> +	 * value 0xFFFF0FF0, defined as DR6_RESERVED, immediately.
>>   	 */
>> -	get_debugreg(dr6, 6);
>>   	set_debugreg(DR6_RESERVED, 6);
>> -	dr6 ^= DR6_RESERVED; /* Flip to positive polarity */
>>     	return dr6;
>>   }
>> @@ -1239,13 +1247,13 @@ static noinstr void exc_debug_user(struct pt_regs *regs, unsigned long dr6)
>>   /* IST stack entry */
>>   DEFINE_IDTENTRY_DEBUG(exc_debug)
>>   {
>> -	exc_debug_kernel(regs, debug_read_clear_dr6());
>> +	exc_debug_kernel(regs, debug_read_reset_dr6());
>>   }
>>     /* User entry, runs on regular task stack */
>>   DEFINE_IDTENTRY_DEBUG_USER(exc_debug)
>>   {
>> -	exc_debug_user(regs, debug_read_clear_dr6());
>> +	exc_debug_user(regs, debug_read_reset_dr6());
>>   }
>>     #ifdef CONFIG_X86_FRED
>> @@ -1264,7 +1272,7 @@ DEFINE_FREDENTRY_DEBUG(exc_debug)
>>   {
>>   	/*
>>   	 * FRED #DB stores DR6 on the stack in the format which
>> -	 * debug_read_clear_dr6() returns for the IDT entry points.
>> +	 * debug_read_reset_dr6() returns for the IDT entry points.
>>   	 */
>>   	unsigned long dr6 = fred_event_data(regs);
>>   @@ -1279,7 +1287,7 @@ DEFINE_FREDENTRY_DEBUG(exc_debug)
>>   /* 32 bit does not have separate entry points. */
>>   DEFINE_IDTENTRY_RAW(exc_debug)
>>   {
>> -	unsigned long dr6 = debug_read_clear_dr6();
>> +	unsigned long dr6 = debug_read_reset_dr6();
>>     	if (user_mode(regs))
>>   		exc_debug_user(regs, dr6);
>

We should, but it isn't a manifest bug so is slightly less urgent.

Re: [PATCH v4 1/2] x86/traps: Initialize DR6 by writing its architectural reset value

[Ethan Zhao]

在 2025/6/23 16:14, H. Peter Anvin 写道:
> On June 22, 2025 11:49:11 PM PDT, Ethan Zhao <haifeng.zhao@linux.intel.com> wrote:
>> 在 2025/6/21 7:15, Xin Li (Intel) 写道:
>>> Initialize DR6 by writing its architectural reset value to avoid
>>> incorrectly zeroing DR6 to clear DR6.BLD at boot time, which leads
>>> to a false bus lock detected warning.
>>>
>>> The Intel SDM says:
>>>
>>>     1) Certain debug exceptions may clear bits 0-3 of DR6.
>>>
>>>     2) BLD induced #DB clears DR6.BLD and any other debug exception
>>>        doesn't modify DR6.BLD.
>>>
>>>     3) RTM induced #DB clears DR6.RTM and any other debug exception
>>>        sets DR6.RTM.
>>>
>>>     To avoid confusion in identifying debug exceptions, debug handlers
>>>     should set DR6.BLD and DR6.RTM, and clear other DR6 bits before
>>>     returning.
>>>
>>> The DR6 architectural reset value 0xFFFF0FF0, already defined as
>>> macro DR6_RESERVED, satisfies these requirements, so just use it to
>>> reinitialize DR6 whenever needed.
>>>
>>> Since clear_all_debug_regs() no longer zeros all debug registers,
>>> rename it to initialize_debug_regs() to better reflect its current
>>> behavior.
>>>
>>> Since debug_read_clear_dr6() no longer clears DR6, rename it to
>>> debug_read_reset_dr6() to better reflect its current behavior.
>>>
>>> Reported-by: Sohil Mehta <sohil.mehta@intel.com>
>>> Link: https://lore.kernel.org/lkml/06e68373-a92b-472e-8fd9-ba548119770c@intel.com/
>>> Fixes: ebb1064e7c2e9 ("x86/traps: Handle #DB for bus lock")
>>> Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>>> Tested-by: Sohil Mehta <sohil.mehta@intel.com>
>>> Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>>> Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
>>> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>>> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
>>> Cc: stable@vger.kernel.org
>>> ---
>>>
>>> Changes in v3:
>>> *) Polish initialize_debug_regs() (PeterZ).
>>> *) Rewrite the comment for DR6_RESERVED definition (Sohil and Sean).
>>> *) Collect TB, RB, AB (PeterZ and Sohil).
>>>
>>> Changes in v2:
>>> *) Use debug register index 6 rather than DR_STATUS (PeterZ and Sean).
>>> *) Move this patch the first of the patch set to ease backporting.
>>> ---
>>>    arch/x86/include/uapi/asm/debugreg.h | 21 ++++++++++++++++-
>>>    arch/x86/kernel/cpu/common.c         | 24 ++++++++------------
>>>    arch/x86/kernel/traps.c              | 34 +++++++++++++++++-----------
>>>    3 files changed, 51 insertions(+), 28 deletions(-)
>>>
>>> diff --git a/arch/x86/include/uapi/asm/debugreg.h b/arch/x86/include/uapi/asm/debugreg.h
>>> index 0007ba077c0c..41da492dfb01 100644
>>> --- a/arch/x86/include/uapi/asm/debugreg.h
>>> +++ b/arch/x86/include/uapi/asm/debugreg.h
>>> @@ -15,7 +15,26 @@
>>>       which debugging register was responsible for the trap.  The other bits
>>>       are either reserved or not of interest to us. */
>>>    -/* Define reserved bits in DR6 which are always set to 1 */
>>> +/*
>>> + * Define bits in DR6 which are set to 1 by default.
>>> + *
>>> + * This is also the DR6 architectural value following Power-up, Reset or INIT.
>>> + *
>>> + * Note, with the introduction of Bus Lock Detection (BLD) and Restricted
>>> + * Transactional Memory (RTM), the DR6 register has been modified:
>>> + *
>>> + * 1) BLD flag (bit 11) is no longer reserved to 1 if the CPU supports
>>> + *    Bus Lock Detection.  The assertion of a bus lock could clear it.
>>> + *
>>> + * 2) RTM flag (bit 16) is no longer reserved to 1 if the CPU supports
>>> + *    restricted transactional memory.  #DB occurred inside an RTM region
>>> + *    could clear it.
>>> + *
>>> + * Apparently, DR6.BLD and DR6.RTM are active low bits.
>>> + *
>>> + * As a result, DR6_RESERVED is an incorrect name now, but it is kept for
>>> + * compatibility.
>>> + */
>>>    #define DR6_RESERVED	(0xFFFF0FF0)
>>>      #define DR_TRAP0	(0x1)		/* db0 */
>>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>>> index 8feb8fd2957a..0f6c280a94f0 100644
>>> --- a/arch/x86/kernel/cpu/common.c
>>> +++ b/arch/x86/kernel/cpu/common.c
>>> @@ -2243,20 +2243,16 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
>>>    #endif
>>>    #endif
>>>    -/*
>>> - * Clear all 6 debug registers:
>>> - */
>>> -static void clear_all_debug_regs(void)
>>> +static void initialize_debug_regs(void)
>>>    {
>>> -	int i;
>>> -
>>> -	for (i = 0; i < 8; i++) {
>>> -		/* Ignore db4, db5 */
>>> -		if ((i == 4) || (i == 5))
>>> -			continue;
>>> -
>>> -		set_debugreg(0, i);
>>> -	}
>>> +	/* Control register first -- to make sure everything is disabled. */
>> In the Figure 19-1. Debug Registers of SDM section 19.2 DEBUG REGISTERS,
>>
>> bit 10, 12, 14, 15 of DR7 are marked as gray (Reversed) and their value are filled as
>>
>> 1, 0, 0,0 ; should we clear them all here ?  I didn't find any other description in the
>>
>> SDM about the result if they are cleaned. of course, this patch doesn't change
>>
>> the behaviour of original DR7 initialization code, no justification needed,
>>
>> just out of curiosity.
>>
>>
>> Thanks,
>>
>> Ethan
>>
>>> +	set_debugreg(0, 7);
>>> +	set_debugreg(DR6_RESERVED, 6);
>>> +	/* dr5 and dr4 don't exist */
>>> +	set_debugreg(0, 3);
>>> +	set_debugreg(0, 2);
>>> +	set_debugreg(0, 1);
>>> +	set_debugreg(0, 0);
>>>    }
>>>      #ifdef CONFIG_KGDB
>>> @@ -2417,7 +2413,7 @@ void cpu_init(void)
>>>      	load_mm_ldt(&init_mm);
>>>    -	clear_all_debug_regs();
>>> +	initialize_debug_regs();
>>>    	dbg_restore_debug_regs();
>>>      	doublefault_init_cpu_tss();
>>> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
>>> index c5c897a86418..36354b470590 100644
>>> --- a/arch/x86/kernel/traps.c
>>> +++ b/arch/x86/kernel/traps.c
>>> @@ -1022,24 +1022,32 @@ static bool is_sysenter_singlestep(struct pt_regs *regs)
>>>    #endif
>>>    }
>>>    -static __always_inline unsigned long debug_read_clear_dr6(void)
>>> +static __always_inline unsigned long debug_read_reset_dr6(void)
>>>    {
>>>    	unsigned long dr6;
>>>    +	get_debugreg(dr6, 6);
>>> +	dr6 ^= DR6_RESERVED; /* Flip to positive polarity */
>>> +
>>>    	/*
>>>    	 * The Intel SDM says:
>>>    	 *
>>> -	 *   Certain debug exceptions may clear bits 0-3. The remaining
>>> -	 *   contents of the DR6 register are never cleared by the
>>> -	 *   processor. To avoid confusion in identifying debug
>>> -	 *   exceptions, debug handlers should clear the register before
>>> -	 *   returning to the interrupted task.
>>> +	 *   Certain debug exceptions may clear bits 0-3 of DR6.
>>> +	 *
>>> +	 *   BLD induced #DB clears DR6.BLD and any other debug
>>> +	 *   exception doesn't modify DR6.BLD.
>>>    	 *
>>> -	 * Keep it simple: clear DR6 immediately.
>>> +	 *   RTM induced #DB clears DR6.RTM and any other debug
>>> +	 *   exception sets DR6.RTM.
>>> +	 *
>>> +	 *   To avoid confusion in identifying debug exceptions,
>>> +	 *   debug handlers should set DR6.BLD and DR6.RTM, and
>>> +	 *   clear other DR6 bits before returning.
>>> +	 *
>>> +	 * Keep it simple: write DR6 with its architectural reset
>>> +	 * value 0xFFFF0FF0, defined as DR6_RESERVED, immediately.
>>>    	 */
>>> -	get_debugreg(dr6, 6);
>>>    	set_debugreg(DR6_RESERVED, 6);
>>> -	dr6 ^= DR6_RESERVED; /* Flip to positive polarity */
>>>      	return dr6;
>>>    }
>>> @@ -1239,13 +1247,13 @@ static noinstr void exc_debug_user(struct pt_regs *regs, unsigned long dr6)
>>>    /* IST stack entry */
>>>    DEFINE_IDTENTRY_DEBUG(exc_debug)
>>>    {
>>> -	exc_debug_kernel(regs, debug_read_clear_dr6());
>>> +	exc_debug_kernel(regs, debug_read_reset_dr6());
>>>    }
>>>      /* User entry, runs on regular task stack */
>>>    DEFINE_IDTENTRY_DEBUG_USER(exc_debug)
>>>    {
>>> -	exc_debug_user(regs, debug_read_clear_dr6());
>>> +	exc_debug_user(regs, debug_read_reset_dr6());
>>>    }
>>>      #ifdef CONFIG_X86_FRED
>>> @@ -1264,7 +1272,7 @@ DEFINE_FREDENTRY_DEBUG(exc_debug)
>>>    {
>>>    	/*
>>>    	 * FRED #DB stores DR6 on the stack in the format which
>>> -	 * debug_read_clear_dr6() returns for the IDT entry points.
>>> +	 * debug_read_reset_dr6() returns for the IDT entry points.
>>>    	 */
>>>    	unsigned long dr6 = fred_event_data(regs);
>>>    @@ -1279,7 +1287,7 @@ DEFINE_FREDENTRY_DEBUG(exc_debug)
>>>    /* 32 bit does not have separate entry points. */
>>>    DEFINE_IDTENTRY_RAW(exc_debug)
>>>    {
>>> -	unsigned long dr6 = debug_read_clear_dr6();
>>> +	unsigned long dr6 = debug_read_reset_dr6();
>>>      	if (user_mode(regs))
>>>    		exc_debug_user(regs, dr6);
> We should, but it isn't a manifest bug so is slightly less urgent.

so far,  yes.


Thanks,

Ethan

-- 
"firm, enduring, strong, and long-lived"

Re: [PATCH v4 1/2] x86/traps: Initialize DR6 by writing its architectural reset value

[Xin Li]

On 6/22/2025 11:49 PM, Ethan Zhao wrote:
> 
> 在 2025/6/21 7:15, Xin Li (Intel) 写道:
>> Initialize DR6 by writing its architectural reset value to avoid
>> incorrectly zeroing DR6 to clear DR6.BLD at boot time, which leads
>> to a false bus lock detected warning.
>>
>> The Intel SDM says:
>>
>>    1) Certain debug exceptions may clear bits 0-3 of DR6.
>>
>>    2) BLD induced #DB clears DR6.BLD and any other debug exception
>>       doesn't modify DR6.BLD.
>>
>>    3) RTM induced #DB clears DR6.RTM and any other debug exception
>>       sets DR6.RTM.
>>
>>    To avoid confusion in identifying debug exceptions, debug handlers
>>    should set DR6.BLD and DR6.RTM, and clear other DR6 bits before
>>    returning.
>>
>> The DR6 architectural reset value 0xFFFF0FF0, already defined as
>> macro DR6_RESERVED, satisfies these requirements, so just use it to
>> reinitialize DR6 whenever needed.
>>
>> Since clear_all_debug_regs() no longer zeros all debug registers,
>> rename it to initialize_debug_regs() to better reflect its current
>> behavior.
>>
>> Since debug_read_clear_dr6() no longer clears DR6, rename it to
>> debug_read_reset_dr6() to better reflect its current behavior.
>>
>> Reported-by: Sohil Mehta <sohil.mehta@intel.com>
>> Link: https://lore.kernel.org/lkml/06e68373-a92b-472e-8fd9- 
>> ba548119770c@intel.com/
>> Fixes: ebb1064e7c2e9 ("x86/traps: Handle #DB for bus lock")
>> Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>> Tested-by: Sohil Mehta <sohil.mehta@intel.com>
>> Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>> Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
>> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
>> Cc: stable@vger.kernel.org
>> ---
>>
>> Changes in v3:
>> *) Polish initialize_debug_regs() (PeterZ).
>> *) Rewrite the comment for DR6_RESERVED definition (Sohil and Sean).
>> *) Collect TB, RB, AB (PeterZ and Sohil).
>>
>> Changes in v2:
>> *) Use debug register index 6 rather than DR_STATUS (PeterZ and Sean).
>> *) Move this patch the first of the patch set to ease backporting.
>> ---
>>   arch/x86/include/uapi/asm/debugreg.h | 21 ++++++++++++++++-
>>   arch/x86/kernel/cpu/common.c         | 24 ++++++++------------
>>   arch/x86/kernel/traps.c              | 34 +++++++++++++++++-----------
>>   3 files changed, 51 insertions(+), 28 deletions(-)
>>
>> diff --git a/arch/x86/include/uapi/asm/debugreg.h b/arch/x86/include/ 
>> uapi/asm/debugreg.h
>> index 0007ba077c0c..41da492dfb01 100644
>> --- a/arch/x86/include/uapi/asm/debugreg.h
>> +++ b/arch/x86/include/uapi/asm/debugreg.h
>> @@ -15,7 +15,26 @@
>>      which debugging register was responsible for the trap.  The other 
>> bits
>>      are either reserved or not of interest to us. */
>> -/* Define reserved bits in DR6 which are always set to 1 */
>> +/*
>> + * Define bits in DR6 which are set to 1 by default.
>> + *
>> + * This is also the DR6 architectural value following Power-up, Reset 
>> or INIT.
>> + *
>> + * Note, with the introduction of Bus Lock Detection (BLD) and 
>> Restricted
>> + * Transactional Memory (RTM), the DR6 register has been modified:
>> + *
>> + * 1) BLD flag (bit 11) is no longer reserved to 1 if the CPU supports
>> + *    Bus Lock Detection.  The assertion of a bus lock could clear it.
>> + *
>> + * 2) RTM flag (bit 16) is no longer reserved to 1 if the CPU supports
>> + *    restricted transactional memory.  #DB occurred inside an RTM 
>> region
>> + *    could clear it.
>> + *
>> + * Apparently, DR6.BLD and DR6.RTM are active low bits.
>> + *
>> + * As a result, DR6_RESERVED is an incorrect name now, but it is kept 
>> for
>> + * compatibility.
>> + */
>>   #define DR6_RESERVED    (0xFFFF0FF0)
>>   #define DR_TRAP0    (0x1)        /* db0 */
>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>> index 8feb8fd2957a..0f6c280a94f0 100644
>> --- a/arch/x86/kernel/cpu/common.c
>> +++ b/arch/x86/kernel/cpu/common.c
>> @@ -2243,20 +2243,16 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
>>   #endif
>>   #endif
>> -/*
>> - * Clear all 6 debug registers:
>> - */
>> -static void clear_all_debug_regs(void)
>> +static void initialize_debug_regs(void)
>>   {
>> -    int i;
>> -
>> -    for (i = 0; i < 8; i++) {
>> -        /* Ignore db4, db5 */
>> -        if ((i == 4) || (i == 5))
>> -            continue;
>> -
>> -        set_debugreg(0, i);
>> -    }
>> +    /* Control register first -- to make sure everything is disabled. */
> 
> In the Figure 19-1. Debug Registers of SDM section 19.2 DEBUG REGISTERS,
> 
> bit 10, 12, 14, 15 of DR7 are marked as gray (Reversed) and their value 
> are filled as
> 
> 1, 0, 0,0 ; should we clear them all here ?  I didn't find any other 
> description in the
> 
> SDM about the result if they are cleaned. of course, this patch doesn't 
> change
> 
> the behaviour of original DR7 initialization code, no justification needed,
> 
> just out of curiosity.

This patch is NOT intended to make any actual change to DR7
initialization.

Please take a look at the second patch of this patch set.

Thanks!
     Xin

> 
>> +    set_debugreg(0, 7);
>> +    set_debugreg(DR6_RESERVED, 6);
>> +    /* dr5 and dr4 don't exist */
>> +    set_debugreg(0, 3);
>> +    set_debugreg(0, 2);
>> +    set_debugreg(0, 1);
>> +    set_debugreg(0, 0);

Re: [PATCH v4 1/2] x86/traps: Initialize DR6 by writing its architectural reset value

[Ethan Zhao]

在 2025/6/24 0:34, Xin Li 写道:
> On 6/22/2025 11:49 PM, Ethan Zhao wrote:
>>
>> 在 2025/6/21 7:15, Xin Li (Intel) 写道:
>>> Initialize DR6 by writing its architectural reset value to avoid
>>> incorrectly zeroing DR6 to clear DR6.BLD at boot time, which leads
>>> to a false bus lock detected warning.
>>>
>>> The Intel SDM says:
>>>
>>>    1) Certain debug exceptions may clear bits 0-3 of DR6.
>>>
>>>    2) BLD induced #DB clears DR6.BLD and any other debug exception
>>>       doesn't modify DR6.BLD.
>>>
>>>    3) RTM induced #DB clears DR6.RTM and any other debug exception
>>>       sets DR6.RTM.
>>>
>>>    To avoid confusion in identifying debug exceptions, debug handlers
>>>    should set DR6.BLD and DR6.RTM, and clear other DR6 bits before
>>>    returning.
>>>
>>> The DR6 architectural reset value 0xFFFF0FF0, already defined as
>>> macro DR6_RESERVED, satisfies these requirements, so just use it to
>>> reinitialize DR6 whenever needed.
>>>
>>> Since clear_all_debug_regs() no longer zeros all debug registers,
>>> rename it to initialize_debug_regs() to better reflect its current
>>> behavior.
>>>
>>> Since debug_read_clear_dr6() no longer clears DR6, rename it to
>>> debug_read_reset_dr6() to better reflect its current behavior.
>>>
>>> Reported-by: Sohil Mehta <sohil.mehta@intel.com>
>>> Link: https://lore.kernel.org/lkml/06e68373-a92b-472e-8fd9- 
>>> ba548119770c@intel.com/
>>> Fixes: ebb1064e7c2e9 ("x86/traps: Handle #DB for bus lock")
>>> Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>>> Tested-by: Sohil Mehta <sohil.mehta@intel.com>
>>> Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>>> Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
>>> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>>> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
>>> Cc: stable@vger.kernel.org
>>> ---
>>>
>>> Changes in v3:
>>> *) Polish initialize_debug_regs() (PeterZ).
>>> *) Rewrite the comment for DR6_RESERVED definition (Sohil and Sean).
>>> *) Collect TB, RB, AB (PeterZ and Sohil).
>>>
>>> Changes in v2:
>>> *) Use debug register index 6 rather than DR_STATUS (PeterZ and Sean).
>>> *) Move this patch the first of the patch set to ease backporting.
>>> ---
>>>   arch/x86/include/uapi/asm/debugreg.h | 21 ++++++++++++++++-
>>>   arch/x86/kernel/cpu/common.c         | 24 ++++++++------------
>>>   arch/x86/kernel/traps.c              | 34 
>>> +++++++++++++++++-----------
>>>   3 files changed, 51 insertions(+), 28 deletions(-)
>>>
>>> diff --git a/arch/x86/include/uapi/asm/debugreg.h 
>>> b/arch/x86/include/ uapi/asm/debugreg.h
>>> index 0007ba077c0c..41da492dfb01 100644
>>> --- a/arch/x86/include/uapi/asm/debugreg.h
>>> +++ b/arch/x86/include/uapi/asm/debugreg.h
>>> @@ -15,7 +15,26 @@
>>>      which debugging register was responsible for the trap. The 
>>> other bits
>>>      are either reserved or not of interest to us. */
>>> -/* Define reserved bits in DR6 which are always set to 1 */
>>> +/*
>>> + * Define bits in DR6 which are set to 1 by default.
>>> + *
>>> + * This is also the DR6 architectural value following Power-up, 
>>> Reset or INIT.
>>> + *
>>> + * Note, with the introduction of Bus Lock Detection (BLD) and 
>>> Restricted
>>> + * Transactional Memory (RTM), the DR6 register has been modified:
>>> + *
>>> + * 1) BLD flag (bit 11) is no longer reserved to 1 if the CPU supports
>>> + *    Bus Lock Detection.  The assertion of a bus lock could clear it.
>>> + *
>>> + * 2) RTM flag (bit 16) is no longer reserved to 1 if the CPU supports
>>> + *    restricted transactional memory.  #DB occurred inside an RTM 
>>> region
>>> + *    could clear it.
>>> + *
>>> + * Apparently, DR6.BLD and DR6.RTM are active low bits.
>>> + *
>>> + * As a result, DR6_RESERVED is an incorrect name now, but it is 
>>> kept for
>>> + * compatibility.
>>> + */
>>>   #define DR6_RESERVED    (0xFFFF0FF0)
>>>   #define DR_TRAP0    (0x1)        /* db0 */
>>> diff --git a/arch/x86/kernel/cpu/common.c 
>>> b/arch/x86/kernel/cpu/common.c
>>> index 8feb8fd2957a..0f6c280a94f0 100644
>>> --- a/arch/x86/kernel/cpu/common.c
>>> +++ b/arch/x86/kernel/cpu/common.c
>>> @@ -2243,20 +2243,16 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
>>>   #endif
>>>   #endif
>>> -/*
>>> - * Clear all 6 debug registers:
>>> - */
>>> -static void clear_all_debug_regs(void)
>>> +static void initialize_debug_regs(void)
>>>   {
>>> -    int i;
>>> -
>>> -    for (i = 0; i < 8; i++) {
>>> -        /* Ignore db4, db5 */
>>> -        if ((i == 4) || (i == 5))
>>> -            continue;
>>> -
>>> -        set_debugreg(0, i);
>>> -    }
>>> +    /* Control register first -- to make sure everything is 
>>> disabled. */
>>
>> In the Figure 19-1. Debug Registers of SDM section 19.2 DEBUG REGISTERS,
>>
>> bit 10, 12, 14, 15 of DR7 are marked as gray (Reversed) and their 
>> value are filled as
>>
>> 1, 0, 0,0 ; should we clear them all here ?  I didn't find any other 
>> description in the
>>
>> SDM about the result if they are cleaned. of course, this patch 
>> doesn't change
>>
>> the behaviour of original DR7 initialization code, no justification 
>> needed,
>>
>> just out of curiosity.
>
> This patch is NOT intended to make any actual change to DR7
> initialization.
>
So far it is okay,  I am just curious why these registers were cleared 
to zero

but the git log history and SDM doesn't give too much consistent clue.

That is 16 years old code.

> Please take a look at the second patch of this patch set.

Looking.


Thanks,

Ethan

>
> Thanks!
>     Xin
>
>>
>>> +    set_debugreg(0, 7);
>>> +    set_debugreg(DR6_RESERVED, 6);
>>> +    /* dr5 and dr4 don't exist */
>>> +    set_debugreg(0, 3);
>>> +    set_debugreg(0, 2);
>>> +    set_debugreg(0, 1);
>>> +    set_debugreg(0, 0);

-- 
"firm, enduring, strong, and long-lived"

Re: [PATCH v4 1/2] x86/traps: Initialize DR6 by writing its architectural reset value

[H. Peter Anvin]

On June 23, 2025 6:19:31 PM PDT, Ethan Zhao <haifeng.zhao@linux.intel.com> wrote:
>
>在 2025/6/24 0:34, Xin Li 写道:
>> On 6/22/2025 11:49 PM, Ethan Zhao wrote:
>>> 
>>> 在 2025/6/21 7:15, Xin Li (Intel) 写道:
>>>> Initialize DR6 by writing its architectural reset value to avoid
>>>> incorrectly zeroing DR6 to clear DR6.BLD at boot time, which leads
>>>> to a false bus lock detected warning.
>>>> 
>>>> The Intel SDM says:
>>>> 
>>>>    1) Certain debug exceptions may clear bits 0-3 of DR6.
>>>> 
>>>>    2) BLD induced #DB clears DR6.BLD and any other debug exception
>>>>       doesn't modify DR6.BLD.
>>>> 
>>>>    3) RTM induced #DB clears DR6.RTM and any other debug exception
>>>>       sets DR6.RTM.
>>>> 
>>>>    To avoid confusion in identifying debug exceptions, debug handlers
>>>>    should set DR6.BLD and DR6.RTM, and clear other DR6 bits before
>>>>    returning.
>>>> 
>>>> The DR6 architectural reset value 0xFFFF0FF0, already defined as
>>>> macro DR6_RESERVED, satisfies these requirements, so just use it to
>>>> reinitialize DR6 whenever needed.
>>>> 
>>>> Since clear_all_debug_regs() no longer zeros all debug registers,
>>>> rename it to initialize_debug_regs() to better reflect its current
>>>> behavior.
>>>> 
>>>> Since debug_read_clear_dr6() no longer clears DR6, rename it to
>>>> debug_read_reset_dr6() to better reflect its current behavior.
>>>> 
>>>> Reported-by: Sohil Mehta <sohil.mehta@intel.com>
>>>> Link: https://lore.kernel.org/lkml/06e68373-a92b-472e-8fd9- ba548119770c@intel.com/
>>>> Fixes: ebb1064e7c2e9 ("x86/traps: Handle #DB for bus lock")
>>>> Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>>>> Tested-by: Sohil Mehta <sohil.mehta@intel.com>
>>>> Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>>>> Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
>>>> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>>>> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
>>>> Cc: stable@vger.kernel.org
>>>> ---
>>>> 
>>>> Changes in v3:
>>>> *) Polish initialize_debug_regs() (PeterZ).
>>>> *) Rewrite the comment for DR6_RESERVED definition (Sohil and Sean).
>>>> *) Collect TB, RB, AB (PeterZ and Sohil).
>>>> 
>>>> Changes in v2:
>>>> *) Use debug register index 6 rather than DR_STATUS (PeterZ and Sean).
>>>> *) Move this patch the first of the patch set to ease backporting.
>>>> ---
>>>>   arch/x86/include/uapi/asm/debugreg.h | 21 ++++++++++++++++-
>>>>   arch/x86/kernel/cpu/common.c         | 24 ++++++++------------
>>>>   arch/x86/kernel/traps.c              | 34 +++++++++++++++++-----------
>>>>   3 files changed, 51 insertions(+), 28 deletions(-)
>>>> 
>>>> diff --git a/arch/x86/include/uapi/asm/debugreg.h b/arch/x86/include/ uapi/asm/debugreg.h
>>>> index 0007ba077c0c..41da492dfb01 100644
>>>> --- a/arch/x86/include/uapi/asm/debugreg.h
>>>> +++ b/arch/x86/include/uapi/asm/debugreg.h
>>>> @@ -15,7 +15,26 @@
>>>>      which debugging register was responsible for the trap. The other bits
>>>>      are either reserved or not of interest to us. */
>>>> -/* Define reserved bits in DR6 which are always set to 1 */
>>>> +/*
>>>> + * Define bits in DR6 which are set to 1 by default.
>>>> + *
>>>> + * This is also the DR6 architectural value following Power-up, Reset or INIT.
>>>> + *
>>>> + * Note, with the introduction of Bus Lock Detection (BLD) and Restricted
>>>> + * Transactional Memory (RTM), the DR6 register has been modified:
>>>> + *
>>>> + * 1) BLD flag (bit 11) is no longer reserved to 1 if the CPU supports
>>>> + *    Bus Lock Detection.  The assertion of a bus lock could clear it.
>>>> + *
>>>> + * 2) RTM flag (bit 16) is no longer reserved to 1 if the CPU supports
>>>> + *    restricted transactional memory.  #DB occurred inside an RTM region
>>>> + *    could clear it.
>>>> + *
>>>> + * Apparently, DR6.BLD and DR6.RTM are active low bits.
>>>> + *
>>>> + * As a result, DR6_RESERVED is an incorrect name now, but it is kept for
>>>> + * compatibility.
>>>> + */
>>>>   #define DR6_RESERVED    (0xFFFF0FF0)
>>>>   #define DR_TRAP0    (0x1)        /* db0 */
>>>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>>>> index 8feb8fd2957a..0f6c280a94f0 100644
>>>> --- a/arch/x86/kernel/cpu/common.c
>>>> +++ b/arch/x86/kernel/cpu/common.c
>>>> @@ -2243,20 +2243,16 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
>>>>   #endif
>>>>   #endif
>>>> -/*
>>>> - * Clear all 6 debug registers:
>>>> - */
>>>> -static void clear_all_debug_regs(void)
>>>> +static void initialize_debug_regs(void)
>>>>   {
>>>> -    int i;
>>>> -
>>>> -    for (i = 0; i < 8; i++) {
>>>> -        /* Ignore db4, db5 */
>>>> -        if ((i == 4) || (i == 5))
>>>> -            continue;
>>>> -
>>>> -        set_debugreg(0, i);
>>>> -    }
>>>> +    /* Control register first -- to make sure everything is disabled. */
>>> 
>>> In the Figure 19-1. Debug Registers of SDM section 19.2 DEBUG REGISTERS,
>>> 
>>> bit 10, 12, 14, 15 of DR7 are marked as gray (Reversed) and their value are filled as
>>> 
>>> 1, 0, 0,0 ; should we clear them all here ?  I didn't find any other description in the
>>> 
>>> SDM about the result if they are cleaned. of course, this patch doesn't change
>>> 
>>> the behaviour of original DR7 initialization code, no justification needed,
>>> 
>>> just out of curiosity.
>> 
>> This patch is NOT intended to make any actual change to DR7
>> initialization.
>> 
>So far it is okay,  I am just curious why these registers were cleared to zero
>
>but the git log history and SDM doesn't give too much consistent clue.
>
>That is 16 years old code.
>
>> Please take a look at the second patch of this patch set.
>
>Looking.
>
>
>Thanks,
>
>Ethan
>
>> 
>> Thanks!
>>     Xin
>> 
>>> 
>>>> +    set_debugreg(0, 7);
>>>> +    set_debugreg(DR6_RESERVED, 6);
>>>> +    /* dr5 and dr4 don't exist */
>>>> +    set_debugreg(0, 3);
>>>> +    set_debugreg(0, 2);
>>>> +    set_debugreg(0, 1);
>>>> +    set_debugreg(0, 0);
>

Older than that.

I believe it dates back from when Linux first got DRx support.

[PATCH v4 2/2] x86/traps: Initialize DR7 by writing its architectural reset value

[Xin Li (Intel)]

Initialize DR7 by writing its architectural reset value to always set
bit 10, which is reserved to '1', when "clearing" DR7 so as not to
trigger unanticipated behavior if said bit is ever unreserved, e.g. as
a feature enabling flag with inverted polarity.

Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Cc: stable@vger.kernel.org
---

Change in v4:
*) Cc stable for backporting, just in case bit 10 of DR7 has become
   unreserved on new hardware, even though clearing it doesn't
   currently cause any real issues (Dave Hansen).

Changes in v3:
*) Reword the changelog using Sean's description.
*) Explain the definition of DR7_FIXED_1 (Sohil).
*) Collect TB, RB, AB (PeterZ, Sohil and Sean).

Changes in v2:
*) Use debug register index 7 rather than DR_CONTROL (PeterZ and Sean).
*) Use DR7_FIXED_1 as the architectural reset value of DR7 (Sean).
---
 arch/x86/include/asm/debugreg.h | 19 +++++++++++++++----
 arch/x86/include/asm/kvm_host.h |  2 +-
 arch/x86/kernel/cpu/common.c    |  2 +-
 arch/x86/kernel/kgdb.c          |  2 +-
 arch/x86/kernel/process_32.c    |  2 +-
 arch/x86/kernel/process_64.c    |  2 +-
 arch/x86/kvm/x86.c              |  4 ++--
 7 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
index 363110e6b2e3..a2c1f2d24b64 100644
--- a/arch/x86/include/asm/debugreg.h
+++ b/arch/x86/include/asm/debugreg.h
@@ -9,6 +9,14 @@
 #include <asm/cpufeature.h>
 #include <asm/msr.h>
 
+/*
+ * Define bits that are always set to 1 in DR7, only bit 10 is
+ * architecturally reserved to '1'.
+ *
+ * This is also the init/reset value for DR7.
+ */
+#define DR7_FIXED_1	0x00000400
+
 DECLARE_PER_CPU(unsigned long, cpu_dr7);
 
 #ifndef CONFIG_PARAVIRT_XXL
@@ -100,8 +108,8 @@ static __always_inline void native_set_debugreg(int regno, unsigned long value)
 
 static inline void hw_breakpoint_disable(void)
 {
-	/* Zero the control register for HW Breakpoint */
-	set_debugreg(0UL, 7);
+	/* Reset the control register for HW Breakpoint */
+	set_debugreg(DR7_FIXED_1, 7);
 
 	/* Zero-out the individual HW breakpoint address registers */
 	set_debugreg(0UL, 0);
@@ -125,9 +133,12 @@ static __always_inline unsigned long local_db_save(void)
 		return 0;
 
 	get_debugreg(dr7, 7);
-	dr7 &= ~0x400; /* architecturally set bit */
+
+	/* Architecturally set bit */
+	dr7 &= ~DR7_FIXED_1;
 	if (dr7)
-		set_debugreg(0, 7);
+		set_debugreg(DR7_FIXED_1, 7);
+
 	/*
 	 * Ensure the compiler doesn't lower the above statements into
 	 * the critical section; disabling breakpoints late would not
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index b4a391929cdb..639d9bcee842 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -31,6 +31,7 @@
 
 #include <asm/apic.h>
 #include <asm/pvclock-abi.h>
+#include <asm/debugreg.h>
 #include <asm/desc.h>
 #include <asm/mtrr.h>
 #include <asm/msr-index.h>
@@ -249,7 +250,6 @@ enum x86_intercept_stage;
 #define DR7_BP_EN_MASK	0x000000ff
 #define DR7_GE		(1 << 9)
 #define DR7_GD		(1 << 13)
-#define DR7_FIXED_1	0x00000400
 #define DR7_VOLATILE	0xffff2bff
 
 #define KVM_GUESTDBG_VALID_MASK \
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 0f6c280a94f0..27125e009847 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2246,7 +2246,7 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
 static void initialize_debug_regs(void)
 {
 	/* Control register first -- to make sure everything is disabled. */
-	set_debugreg(0, 7);
+	set_debugreg(DR7_FIXED_1, 7);
 	set_debugreg(DR6_RESERVED, 6);
 	/* dr5 and dr4 don't exist */
 	set_debugreg(0, 3);
diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index 102641fd2172..8b1a9733d13e 100644
--- a/arch/x86/kernel/kgdb.c
+++ b/arch/x86/kernel/kgdb.c
@@ -385,7 +385,7 @@ static void kgdb_disable_hw_debug(struct pt_regs *regs)
 	struct perf_event *bp;
 
 	/* Disable hardware debugging while we are in kgdb: */
-	set_debugreg(0UL, 7);
+	set_debugreg(DR7_FIXED_1, 7);
 	for (i = 0; i < HBP_NUM; i++) {
 		if (!breakinfo[i].enabled)
 			continue;
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index a10e180cbf23..3ef15c2f152f 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -93,7 +93,7 @@ void __show_regs(struct pt_regs *regs, enum show_regs_mode mode,
 
 	/* Only print out debug registers if they are in their non-default state. */
 	if ((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
-	    (d6 == DR6_RESERVED) && (d7 == 0x400))
+	    (d6 == DR6_RESERVED) && (d7 == DR7_FIXED_1))
 		return;
 
 	printk("%sDR0: %08lx DR1: %08lx DR2: %08lx DR3: %08lx\n",
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 8d6cf25127aa..b972bf72fb8b 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -133,7 +133,7 @@ void __show_regs(struct pt_regs *regs, enum show_regs_mode mode,
 
 	/* Only print out debug registers if they are in their non-default state. */
 	if (!((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
-	    (d6 == DR6_RESERVED) && (d7 == 0x400))) {
+	    (d6 == DR6_RESERVED) && (d7 == DR7_FIXED_1))) {
 		printk("%sDR0: %016lx DR1: %016lx DR2: %016lx\n",
 		       log_lvl, d0, d1, d2);
 		printk("%sDR3: %016lx DR6: %016lx DR7: %016lx\n",
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b58a74c1722d..a9d992d5652f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11035,7 +11035,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 
 	if (unlikely(vcpu->arch.switch_db_regs &&
 		     !(vcpu->arch.switch_db_regs & KVM_DEBUGREG_AUTO_SWITCH))) {
-		set_debugreg(0, 7);
+		set_debugreg(DR7_FIXED_1, 7);
 		set_debugreg(vcpu->arch.eff_db[0], 0);
 		set_debugreg(vcpu->arch.eff_db[1], 1);
 		set_debugreg(vcpu->arch.eff_db[2], 2);
@@ -11044,7 +11044,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT))
 			kvm_x86_call(set_dr6)(vcpu, vcpu->arch.dr6);
 	} else if (unlikely(hw_breakpoint_active())) {
-		set_debugreg(0, 7);
+		set_debugreg(DR7_FIXED_1, 7);
 	}
 
 	vcpu->arch.host_debugctl = get_debugctlmsr();
-- 
2.49.0

Re: [PATCH v4 2/2] x86/traps: Initialize DR7 by writing its architectural reset value

[Ethan Zhao]

在 2025/6/21 7:15, Xin Li (Intel) 写道:
> Initialize DR7 by writing its architectural reset value to always set
> bit 10, which is reserved to '1', when "clearing" DR7 so as not to
> trigger unanticipated behavior if said bit is ever unreserved, e.g. as
> a feature enabling flag with inverted polarity.
>
> Tested-by: Sohil Mehta <sohil.mehta@intel.com>
> Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
> Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Acked-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
> Cc: stable@vger.kernel.org
> ---
>
> Change in v4:
> *) Cc stable for backporting, just in case bit 10 of DR7 has become
>     unreserved on new hardware, even though clearing it doesn't
>     currently cause any real issues (Dave Hansen).
>
> Changes in v3:
> *) Reword the changelog using Sean's description.
> *) Explain the definition of DR7_FIXED_1 (Sohil).
> *) Collect TB, RB, AB (PeterZ, Sohil and Sean).
>
> Changes in v2:
> *) Use debug register index 7 rather than DR_CONTROL (PeterZ and Sean).
> *) Use DR7_FIXED_1 as the architectural reset value of DR7 (Sean).
> ---
>   arch/x86/include/asm/debugreg.h | 19 +++++++++++++++----
>   arch/x86/include/asm/kvm_host.h |  2 +-
>   arch/x86/kernel/cpu/common.c    |  2 +-
>   arch/x86/kernel/kgdb.c          |  2 +-
>   arch/x86/kernel/process_32.c    |  2 +-
>   arch/x86/kernel/process_64.c    |  2 +-
>   arch/x86/kvm/x86.c              |  4 ++--
>   7 files changed, 22 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
> index 363110e6b2e3..a2c1f2d24b64 100644
> --- a/arch/x86/include/asm/debugreg.h
> +++ b/arch/x86/include/asm/debugreg.h
> @@ -9,6 +9,14 @@
>   #include <asm/cpufeature.h>
>   #include <asm/msr.h>
>   
> +/*
> + * Define bits that are always set to 1 in DR7, only bit 10 is
> + * architecturally reserved to '1'.
> + *
> + * This is also the init/reset value for DR7.
> + */
> +#define DR7_FIXED_1	0x00000400
> +
>   DECLARE_PER_CPU(unsigned long, cpu_dr7);
>   
>   #ifndef CONFIG_PARAVIRT_XXL
> @@ -100,8 +108,8 @@ static __always_inline void native_set_debugreg(int regno, unsigned long value)
>   
>   static inline void hw_breakpoint_disable(void)
>   {
> -	/* Zero the control register for HW Breakpoint */
> -	set_debugreg(0UL, 7);
> +	/* Reset the control register for HW Breakpoint */
> +	set_debugreg(DR7_FIXED_1, 7);

Given you have it be adhere to SDM about the DR7 reversed bits setting,

then no reason to leave patch[1/2] to set_debugreg(0, 7) alone.

did I miss something here ?


Thanks,

Ethan


>   
>   	/* Zero-out the individual HW breakpoint address registers */
>   	set_debugreg(0UL, 0);
> @@ -125,9 +133,12 @@ static __always_inline unsigned long local_db_save(void)
>   		return 0;
>   
>   	get_debugreg(dr7, 7);
> -	dr7 &= ~0x400; /* architecturally set bit */
> +
> +	/* Architecturally set bit */
> +	dr7 &= ~DR7_FIXED_1;
>   	if (dr7)
> -		set_debugreg(0, 7);
> +		set_debugreg(DR7_FIXED_1, 7);
> +
>   	/*
>   	 * Ensure the compiler doesn't lower the above statements into
>   	 * the critical section; disabling breakpoints late would not
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index b4a391929cdb..639d9bcee842 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -31,6 +31,7 @@
>   
>   #include <asm/apic.h>
>   #include <asm/pvclock-abi.h>
> +#include <asm/debugreg.h>
>   #include <asm/desc.h>
>   #include <asm/mtrr.h>
>   #include <asm/msr-index.h>
> @@ -249,7 +250,6 @@ enum x86_intercept_stage;
>   #define DR7_BP_EN_MASK	0x000000ff
>   #define DR7_GE		(1 << 9)
>   #define DR7_GD		(1 << 13)
> -#define DR7_FIXED_1	0x00000400
>   #define DR7_VOLATILE	0xffff2bff
>   
>   #define KVM_GUESTDBG_VALID_MASK \
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 0f6c280a94f0..27125e009847 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2246,7 +2246,7 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
>   static void initialize_debug_regs(void)
>   {
>   	/* Control register first -- to make sure everything is disabled. */
> -	set_debugreg(0, 7);
> +	set_debugreg(DR7_FIXED_1, 7);
>   	set_debugreg(DR6_RESERVED, 6);
>   	/* dr5 and dr4 don't exist */
>   	set_debugreg(0, 3);
> diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
> index 102641fd2172..8b1a9733d13e 100644
> --- a/arch/x86/kernel/kgdb.c
> +++ b/arch/x86/kernel/kgdb.c
> @@ -385,7 +385,7 @@ static void kgdb_disable_hw_debug(struct pt_regs *regs)
>   	struct perf_event *bp;
>   
>   	/* Disable hardware debugging while we are in kgdb: */
> -	set_debugreg(0UL, 7);
> +	set_debugreg(DR7_FIXED_1, 7);
>   	for (i = 0; i < HBP_NUM; i++) {
>   		if (!breakinfo[i].enabled)
>   			continue;
> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> index a10e180cbf23..3ef15c2f152f 100644
> --- a/arch/x86/kernel/process_32.c
> +++ b/arch/x86/kernel/process_32.c
> @@ -93,7 +93,7 @@ void __show_regs(struct pt_regs *regs, enum show_regs_mode mode,
>   
>   	/* Only print out debug registers if they are in their non-default state. */
>   	if ((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
> -	    (d6 == DR6_RESERVED) && (d7 == 0x400))
> +	    (d6 == DR6_RESERVED) && (d7 == DR7_FIXED_1))
>   		return;
>   
>   	printk("%sDR0: %08lx DR1: %08lx DR2: %08lx DR3: %08lx\n",
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index 8d6cf25127aa..b972bf72fb8b 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -133,7 +133,7 @@ void __show_regs(struct pt_regs *regs, enum show_regs_mode mode,
>   
>   	/* Only print out debug registers if they are in their non-default state. */
>   	if (!((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
> -	    (d6 == DR6_RESERVED) && (d7 == 0x400))) {
> +	    (d6 == DR6_RESERVED) && (d7 == DR7_FIXED_1))) {
>   		printk("%sDR0: %016lx DR1: %016lx DR2: %016lx\n",
>   		       log_lvl, d0, d1, d2);
>   		printk("%sDR3: %016lx DR6: %016lx DR7: %016lx\n",
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b58a74c1722d..a9d992d5652f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11035,7 +11035,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>   
>   	if (unlikely(vcpu->arch.switch_db_regs &&
>   		     !(vcpu->arch.switch_db_regs & KVM_DEBUGREG_AUTO_SWITCH))) {
> -		set_debugreg(0, 7);
> +		set_debugreg(DR7_FIXED_1, 7);
>   		set_debugreg(vcpu->arch.eff_db[0], 0);
>   		set_debugreg(vcpu->arch.eff_db[1], 1);
>   		set_debugreg(vcpu->arch.eff_db[2], 2);
> @@ -11044,7 +11044,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>   		if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT))
>   			kvm_x86_call(set_dr6)(vcpu, vcpu->arch.dr6);
>   	} else if (unlikely(hw_breakpoint_active())) {
> -		set_debugreg(0, 7);
> +		set_debugreg(DR7_FIXED_1, 7);
>   	}
>   
>   	vcpu->arch.host_debugctl = get_debugctlmsr();

-- 
"firm, enduring, strong, and long-lived"

Re: [PATCH v4 2/2] x86/traps: Initialize DR7 by writing its architectural reset value

[H. Peter Anvin]

On June 23, 2025 6:41:07 PM PDT, Ethan Zhao <haifeng.zhao@linux.intel.com> wrote:
>
>在 2025/6/21 7:15, Xin Li (Intel) 写道:
>> Initialize DR7 by writing its architectural reset value to always set
>> bit 10, which is reserved to '1', when "clearing" DR7 so as not to
>> trigger unanticipated behavior if said bit is ever unreserved, e.g. as
>> a feature enabling flag with inverted polarity.
>> 
>> Tested-by: Sohil Mehta <sohil.mehta@intel.com>
>> Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>> Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
>> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>> Acked-by: Sean Christopherson <seanjc@google.com>
>> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
>> Cc: stable@vger.kernel.org
>> ---
>> 
>> Change in v4:
>> *) Cc stable for backporting, just in case bit 10 of DR7 has become
>>     unreserved on new hardware, even though clearing it doesn't
>>     currently cause any real issues (Dave Hansen).
>> 
>> Changes in v3:
>> *) Reword the changelog using Sean's description.
>> *) Explain the definition of DR7_FIXED_1 (Sohil).
>> *) Collect TB, RB, AB (PeterZ, Sohil and Sean).
>> 
>> Changes in v2:
>> *) Use debug register index 7 rather than DR_CONTROL (PeterZ and Sean).
>> *) Use DR7_FIXED_1 as the architectural reset value of DR7 (Sean).
>> ---
>>   arch/x86/include/asm/debugreg.h | 19 +++++++++++++++----
>>   arch/x86/include/asm/kvm_host.h |  2 +-
>>   arch/x86/kernel/cpu/common.c    |  2 +-
>>   arch/x86/kernel/kgdb.c          |  2 +-
>>   arch/x86/kernel/process_32.c    |  2 +-
>>   arch/x86/kernel/process_64.c    |  2 +-
>>   arch/x86/kvm/x86.c              |  4 ++--
>>   7 files changed, 22 insertions(+), 11 deletions(-)
>> 
>> diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
>> index 363110e6b2e3..a2c1f2d24b64 100644
>> --- a/arch/x86/include/asm/debugreg.h
>> +++ b/arch/x86/include/asm/debugreg.h
>> @@ -9,6 +9,14 @@
>>   #include <asm/cpufeature.h>
>>   #include <asm/msr.h>
>>   +/*
>> + * Define bits that are always set to 1 in DR7, only bit 10 is
>> + * architecturally reserved to '1'.
>> + *
>> + * This is also the init/reset value for DR7.
>> + */
>> +#define DR7_FIXED_1	0x00000400
>> +
>>   DECLARE_PER_CPU(unsigned long, cpu_dr7);
>>     #ifndef CONFIG_PARAVIRT_XXL
>> @@ -100,8 +108,8 @@ static __always_inline void native_set_debugreg(int regno, unsigned long value)
>>     static inline void hw_breakpoint_disable(void)
>>   {
>> -	/* Zero the control register for HW Breakpoint */
>> -	set_debugreg(0UL, 7);
>> +	/* Reset the control register for HW Breakpoint */
>> +	set_debugreg(DR7_FIXED_1, 7);
>
>Given you have it be adhere to SDM about the DR7 reversed bits setting,
>
>then no reason to leave patch[1/2] to set_debugreg(0, 7) alone.
>
>did I miss something here ?
>
>
>Thanks,
>
>Ethan
>
>
>>     	/* Zero-out the individual HW breakpoint address registers */
>>   	set_debugreg(0UL, 0);
>> @@ -125,9 +133,12 @@ static __always_inline unsigned long local_db_save(void)
>>   		return 0;
>>     	get_debugreg(dr7, 7);
>> -	dr7 &= ~0x400; /* architecturally set bit */
>> +
>> +	/* Architecturally set bit */
>> +	dr7 &= ~DR7_FIXED_1;
>>   	if (dr7)
>> -		set_debugreg(0, 7);
>> +		set_debugreg(DR7_FIXED_1, 7);
>> +
>>   	/*
>>   	 * Ensure the compiler doesn't lower the above statements into
>>   	 * the critical section; disabling breakpoints late would not
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index b4a391929cdb..639d9bcee842 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -31,6 +31,7 @@
>>     #include <asm/apic.h>
>>   #include <asm/pvclock-abi.h>
>> +#include <asm/debugreg.h>
>>   #include <asm/desc.h>
>>   #include <asm/mtrr.h>
>>   #include <asm/msr-index.h>
>> @@ -249,7 +250,6 @@ enum x86_intercept_stage;
>>   #define DR7_BP_EN_MASK	0x000000ff
>>   #define DR7_GE		(1 << 9)
>>   #define DR7_GD		(1 << 13)
>> -#define DR7_FIXED_1	0x00000400
>>   #define DR7_VOLATILE	0xffff2bff
>>     #define KVM_GUESTDBG_VALID_MASK \
>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>> index 0f6c280a94f0..27125e009847 100644
>> --- a/arch/x86/kernel/cpu/common.c
>> +++ b/arch/x86/kernel/cpu/common.c
>> @@ -2246,7 +2246,7 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
>>   static void initialize_debug_regs(void)
>>   {
>>   	/* Control register first -- to make sure everything is disabled. */
>> -	set_debugreg(0, 7);
>> +	set_debugreg(DR7_FIXED_1, 7);
>>   	set_debugreg(DR6_RESERVED, 6);
>>   	/* dr5 and dr4 don't exist */
>>   	set_debugreg(0, 3);
>> diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
>> index 102641fd2172..8b1a9733d13e 100644
>> --- a/arch/x86/kernel/kgdb.c
>> +++ b/arch/x86/kernel/kgdb.c
>> @@ -385,7 +385,7 @@ static void kgdb_disable_hw_debug(struct pt_regs *regs)
>>   	struct perf_event *bp;
>>     	/* Disable hardware debugging while we are in kgdb: */
>> -	set_debugreg(0UL, 7);
>> +	set_debugreg(DR7_FIXED_1, 7);
>>   	for (i = 0; i < HBP_NUM; i++) {
>>   		if (!breakinfo[i].enabled)
>>   			continue;
>> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
>> index a10e180cbf23..3ef15c2f152f 100644
>> --- a/arch/x86/kernel/process_32.c
>> +++ b/arch/x86/kernel/process_32.c
>> @@ -93,7 +93,7 @@ void __show_regs(struct pt_regs *regs, enum show_regs_mode mode,
>>     	/* Only print out debug registers if they are in their non-default state. */
>>   	if ((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
>> -	    (d6 == DR6_RESERVED) && (d7 == 0x400))
>> +	    (d6 == DR6_RESERVED) && (d7 == DR7_FIXED_1))
>>   		return;
>>     	printk("%sDR0: %08lx DR1: %08lx DR2: %08lx DR3: %08lx\n",
>> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
>> index 8d6cf25127aa..b972bf72fb8b 100644
>> --- a/arch/x86/kernel/process_64.c
>> +++ b/arch/x86/kernel/process_64.c
>> @@ -133,7 +133,7 @@ void __show_regs(struct pt_regs *regs, enum show_regs_mode mode,
>>     	/* Only print out debug registers if they are in their non-default state. */
>>   	if (!((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
>> -	    (d6 == DR6_RESERVED) && (d7 == 0x400))) {
>> +	    (d6 == DR6_RESERVED) && (d7 == DR7_FIXED_1))) {
>>   		printk("%sDR0: %016lx DR1: %016lx DR2: %016lx\n",
>>   		       log_lvl, d0, d1, d2);
>>   		printk("%sDR3: %016lx DR6: %016lx DR7: %016lx\n",
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index b58a74c1722d..a9d992d5652f 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -11035,7 +11035,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>     	if (unlikely(vcpu->arch.switch_db_regs &&
>>   		     !(vcpu->arch.switch_db_regs & KVM_DEBUGREG_AUTO_SWITCH))) {
>> -		set_debugreg(0, 7);
>> +		set_debugreg(DR7_FIXED_1, 7);
>>   		set_debugreg(vcpu->arch.eff_db[0], 0);
>>   		set_debugreg(vcpu->arch.eff_db[1], 1);
>>   		set_debugreg(vcpu->arch.eff_db[2], 2);
>> @@ -11044,7 +11044,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>   		if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT))
>>   			kvm_x86_call(set_dr6)(vcpu, vcpu->arch.dr6);
>>   	} else if (unlikely(hw_breakpoint_active())) {
>> -		set_debugreg(0, 7);
>> +		set_debugreg(DR7_FIXED_1, 7);
>>   	}
>>     	vcpu->arch.host_debugctl = get_debugctlmsr();
>

It's split up for the benefit of the stable maintainers.

Re: [PATCH v4 2/2] x86/traps: Initialize DR7 by writing its architectural reset value

[Ethan Zhao]

在 2025/6/24 9:53, H. Peter Anvin 写道:
> On June 23, 2025 6:41:07 PM PDT, Ethan Zhao <haifeng.zhao@linux.intel.com> wrote:
>> 在 2025/6/21 7:15, Xin Li (Intel) 写道:
>>> Initialize DR7 by writing its architectural reset value to always set
>>> bit 10, which is reserved to '1', when "clearing" DR7 so as not to
>>> trigger unanticipated behavior if said bit is ever unreserved, e.g. as
>>> a feature enabling flag with inverted polarity.
>>>
>>> Tested-by: Sohil Mehta <sohil.mehta@intel.com>
>>> Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>>> Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
>>> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>>> Acked-by: Sean Christopherson <seanjc@google.com>
>>> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
>>> Cc: stable@vger.kernel.org
>>> ---
>>>
>>> Change in v4:
>>> *) Cc stable for backporting, just in case bit 10 of DR7 has become
>>>      unreserved on new hardware, even though clearing it doesn't
>>>      currently cause any real issues (Dave Hansen).
>>>
>>> Changes in v3:
>>> *) Reword the changelog using Sean's description.
>>> *) Explain the definition of DR7_FIXED_1 (Sohil).
>>> *) Collect TB, RB, AB (PeterZ, Sohil and Sean).
>>>
>>> Changes in v2:
>>> *) Use debug register index 7 rather than DR_CONTROL (PeterZ and Sean).
>>> *) Use DR7_FIXED_1 as the architectural reset value of DR7 (Sean).
>>> ---
>>>    arch/x86/include/asm/debugreg.h | 19 +++++++++++++++----
>>>    arch/x86/include/asm/kvm_host.h |  2 +-
>>>    arch/x86/kernel/cpu/common.c    |  2 +-
>>>    arch/x86/kernel/kgdb.c          |  2 +-
>>>    arch/x86/kernel/process_32.c    |  2 +-
>>>    arch/x86/kernel/process_64.c    |  2 +-
>>>    arch/x86/kvm/x86.c              |  4 ++--
>>>    7 files changed, 22 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
>>> index 363110e6b2e3..a2c1f2d24b64 100644
>>> --- a/arch/x86/include/asm/debugreg.h
>>> +++ b/arch/x86/include/asm/debugreg.h
>>> @@ -9,6 +9,14 @@
>>>    #include <asm/cpufeature.h>
>>>    #include <asm/msr.h>
>>>    +/*
>>> + * Define bits that are always set to 1 in DR7, only bit 10 is
>>> + * architecturally reserved to '1'.
>>> + *
>>> + * This is also the init/reset value for DR7.
>>> + */
>>> +#define DR7_FIXED_1	0x00000400
>>> +
>>>    DECLARE_PER_CPU(unsigned long, cpu_dr7);
>>>      #ifndef CONFIG_PARAVIRT_XXL
>>> @@ -100,8 +108,8 @@ static __always_inline void native_set_debugreg(int regno, unsigned long value)
>>>      static inline void hw_breakpoint_disable(void)
>>>    {
>>> -	/* Zero the control register for HW Breakpoint */
>>> -	set_debugreg(0UL, 7);
>>> +	/* Reset the control register for HW Breakpoint */
>>> +	set_debugreg(DR7_FIXED_1, 7);
>> Given you have it be adhere to SDM about the DR7 reversed bits setting,
>>
>> then no reason to leave patch[1/2] to set_debugreg(0, 7) alone.
>>
>> did I miss something here ?
>>
>>
>> Thanks,
>>
>> Ethan
>>
>>
>>>      	/* Zero-out the individual HW breakpoint address registers */
>>>    	set_debugreg(0UL, 0);
>>> @@ -125,9 +133,12 @@ static __always_inline unsigned long local_db_save(void)
>>>    		return 0;
>>>      	get_debugreg(dr7, 7);
>>> -	dr7 &= ~0x400; /* architecturally set bit */
>>> +
>>> +	/* Architecturally set bit */
>>> +	dr7 &= ~DR7_FIXED_1;
>>>    	if (dr7)
>>> -		set_debugreg(0, 7);
>>> +		set_debugreg(DR7_FIXED_1, 7);
>>> +
>>>    	/*
>>>    	 * Ensure the compiler doesn't lower the above statements into
>>>    	 * the critical section; disabling breakpoints late would not
>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>>> index b4a391929cdb..639d9bcee842 100644
>>> --- a/arch/x86/include/asm/kvm_host.h
>>> +++ b/arch/x86/include/asm/kvm_host.h
>>> @@ -31,6 +31,7 @@
>>>      #include <asm/apic.h>
>>>    #include <asm/pvclock-abi.h>
>>> +#include <asm/debugreg.h>
>>>    #include <asm/desc.h>
>>>    #include <asm/mtrr.h>
>>>    #include <asm/msr-index.h>
>>> @@ -249,7 +250,6 @@ enum x86_intercept_stage;
>>>    #define DR7_BP_EN_MASK	0x000000ff
>>>    #define DR7_GE		(1 << 9)
>>>    #define DR7_GD		(1 << 13)
>>> -#define DR7_FIXED_1	0x00000400
>>>    #define DR7_VOLATILE	0xffff2bff
>>>      #define KVM_GUESTDBG_VALID_MASK \
>>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>>> index 0f6c280a94f0..27125e009847 100644
>>> --- a/arch/x86/kernel/cpu/common.c
>>> +++ b/arch/x86/kernel/cpu/common.c
>>> @@ -2246,7 +2246,7 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
>>>    static void initialize_debug_regs(void)
>>>    {
>>>    	/* Control register first -- to make sure everything is disabled. */
>>> -	set_debugreg(0, 7);
>>> +	set_debugreg(DR7_FIXED_1, 7);
>>>    	set_debugreg(DR6_RESERVED, 6);
>>>    	/* dr5 and dr4 don't exist */
>>>    	set_debugreg(0, 3);
>>> diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
>>> index 102641fd2172..8b1a9733d13e 100644
>>> --- a/arch/x86/kernel/kgdb.c
>>> +++ b/arch/x86/kernel/kgdb.c
>>> @@ -385,7 +385,7 @@ static void kgdb_disable_hw_debug(struct pt_regs *regs)
>>>    	struct perf_event *bp;
>>>      	/* Disable hardware debugging while we are in kgdb: */
>>> -	set_debugreg(0UL, 7);
>>> +	set_debugreg(DR7_FIXED_1, 7);
>>>    	for (i = 0; i < HBP_NUM; i++) {
>>>    		if (!breakinfo[i].enabled)
>>>    			continue;
>>> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
>>> index a10e180cbf23..3ef15c2f152f 100644
>>> --- a/arch/x86/kernel/process_32.c
>>> +++ b/arch/x86/kernel/process_32.c
>>> @@ -93,7 +93,7 @@ void __show_regs(struct pt_regs *regs, enum show_regs_mode mode,
>>>      	/* Only print out debug registers if they are in their non-default state. */
>>>    	if ((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
>>> -	    (d6 == DR6_RESERVED) && (d7 == 0x400))
>>> +	    (d6 == DR6_RESERVED) && (d7 == DR7_FIXED_1))
>>>    		return;
>>>      	printk("%sDR0: %08lx DR1: %08lx DR2: %08lx DR3: %08lx\n",
>>> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
>>> index 8d6cf25127aa..b972bf72fb8b 100644
>>> --- a/arch/x86/kernel/process_64.c
>>> +++ b/arch/x86/kernel/process_64.c
>>> @@ -133,7 +133,7 @@ void __show_regs(struct pt_regs *regs, enum show_regs_mode mode,
>>>      	/* Only print out debug registers if they are in their non-default state. */
>>>    	if (!((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
>>> -	    (d6 == DR6_RESERVED) && (d7 == 0x400))) {
>>> +	    (d6 == DR6_RESERVED) && (d7 == DR7_FIXED_1))) {
>>>    		printk("%sDR0: %016lx DR1: %016lx DR2: %016lx\n",
>>>    		       log_lvl, d0, d1, d2);
>>>    		printk("%sDR3: %016lx DR6: %016lx DR7: %016lx\n",
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index b58a74c1722d..a9d992d5652f 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -11035,7 +11035,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>>      	if (unlikely(vcpu->arch.switch_db_regs &&
>>>    		     !(vcpu->arch.switch_db_regs & KVM_DEBUGREG_AUTO_SWITCH))) {
>>> -		set_debugreg(0, 7);
>>> +		set_debugreg(DR7_FIXED_1, 7);
>>>    		set_debugreg(vcpu->arch.eff_db[0], 0);
>>>    		set_debugreg(vcpu->arch.eff_db[1], 1);
>>>    		set_debugreg(vcpu->arch.eff_db[2], 2);
>>> @@ -11044,7 +11044,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>>    		if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT))
>>>    			kvm_x86_call(set_dr6)(vcpu, vcpu->arch.dr6);
>>>    	} else if (unlikely(hw_breakpoint_active())) {
>>> -		set_debugreg(0, 7);
>>> +		set_debugreg(DR7_FIXED_1, 7);
>>>    	}
>>>      	vcpu->arch.host_debugctl = get_debugctlmsr();
> It's split up for the benefit of the stable maintainers.

Undeniably split, as observed. :)


Thanks,

Ethan

>
-- 
"firm, enduring, strong, and long-lived"