[PATCH v4 0/4] x86/speculation: Disable IBRS when idle

[PATCH v4 0/4] x86/speculation: Disable IBRS when idle

[Waiman Long]

 v4:
  - Add a new __update_spec_ctrl() helper in patch 1.
  - Rebased to the latest linux kernel.

 v3:
  - Drop patches 1 ("x86/speculation: Provide a debugfs file to dump
    SPEC_CTRL MSRs") and 5 ("x86/idle: Disable IBRS entering mwait idle
    and enable it on wakeup") for now.
  - Drop the MSR restoration code in ("x86/idle: Disable IBRS when cpu
    is offline") as native_play_dead() does not return.
  - For patch ("intel_idle: Add ibrs_off module parameter to force
    disable IBRS"), change the name from "no_ibrs" to "ibrs_off" and
    document the new parameter in intel_idle.rst.

For Intel processors that need to turn on IBRS to protect against
Spectre v2 and Retbleed, the IBRS bit in the SPEC_CTRL MSR affects
the performance of the whole core even if only one thread is turning
it on when running in the kernel. For user space heavy applications,
the performance impact of occasionally turning IBRS on during syscalls
shouldn't be significant. Unfortunately, that is not the case when the
sibling thread is idling in the kernel. In that case, the performance
impact can be significant.

When DPDK is running on an isolated CPU thread processing network packets
in user space while its sibling thread is idle. The performance of the
busy DPDK thread with IBRS on and off in the sibling idle thread are:

                                IBRS on         IBRS off
                                -------         --------
  packets/second:                  7.8M           10.4M
  avg tsc cycles/packet:         282.26          209.86

This is a 25% performance degradation. The test system is a Intel Xeon
4114 CPU @ 2.20GHz.

Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle")
disables IBRS when the CPU enters long idle (C6 or below). However, there
are existing users out there who have set "intel_idle.max_cstate=1"
to decrease latency. Those users won't be able to benefit from this
commit. This patch series extends this commit by providing a new
"intel_idle.ibrs_off" module parameter to force disable IBRS even when
"intel_idle.max_cstate=1" at the expense of increased IRQ response
latency. It also includes a commit to allow the disabling of IBRS when
a CPU becomes offline.


Waiman Long (4):
  x86/speculation: Add __update_spec_ctrl() helper
  x86/idle: Disable IBRS when cpu is offline
  intel_idle: Use __update_spec_ctrl() in intel_idle_ibrs()
  intel_idle: Add ibrs_off module parameter to force disable IBRS

 Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++-
 arch/x86/include/asm/nospec-branch.h        | 11 ++++++++++-
 arch/x86/kernel/smpboot.c                   |  8 ++++++++
 drivers/idle/intel_idle.c                   | 18 ++++++++++++++----
 4 files changed, 48 insertions(+), 6 deletions(-)

-- 
2.31.1

[PATCH v4 1/4] x86/speculation: Add __update_spec_ctrl() helper

[Waiman Long]

Add a new __update_spec_ctrl() helper which is a variant of
update_spec_ctrl() that can be used in a noinstr function.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 arch/x86/include/asm/nospec-branch.h | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 55388c9f7601..1d363fcea207 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -9,7 +9,7 @@
 
 #include <asm/alternative.h>
 #include <asm/cpufeatures.h>
-#include <asm/msr-index.h>
+#include <asm/msr.h>
 #include <asm/unwind_hints.h>
 #include <asm/percpu.h>
 #include <asm/current.h>
@@ -488,6 +488,15 @@ DECLARE_PER_CPU(u64, x86_spec_ctrl_current);
 extern void update_spec_ctrl_cond(u64 val);
 extern u64 spec_ctrl_current(void);
 
+/*
+ * This can be used in noinstr function.
+ */
+static __always_inline void __update_spec_ctrl(u64 val)
+{
+	__this_cpu_write(x86_spec_ctrl_current, val);
+	native_wrmsrl(MSR_IA32_SPEC_CTRL, val);
+}
+
 /*
  * With retpoline, we must use IBRS to restrict branch prediction
  * before calling into firmware.
-- 
2.31.1

Re: [PATCH v4 1/4] x86/speculation: Add __update_spec_ctrl() helper

[Peter Zijlstra]

On Tue, Jun 27, 2023 at 10:25:51PM -0400, Waiman Long wrote:
> Add a new __update_spec_ctrl() helper which is a variant of
> update_spec_ctrl() that can be used in a noinstr function.
> 
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  arch/x86/include/asm/nospec-branch.h | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 55388c9f7601..1d363fcea207 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -9,7 +9,7 @@
>  
>  #include <asm/alternative.h>
>  #include <asm/cpufeatures.h>
> -#include <asm/msr-index.h>
> +#include <asm/msr.h>
>  #include <asm/unwind_hints.h>
>  #include <asm/percpu.h>
>  #include <asm/current.h>
> @@ -488,6 +488,15 @@ DECLARE_PER_CPU(u64, x86_spec_ctrl_current);
>  extern void update_spec_ctrl_cond(u64 val);
>  extern u64 spec_ctrl_current(void);
>  
> +/*
> + * This can be used in noinstr function.
> + */
> +static __always_inline void __update_spec_ctrl(u64 val)
> +{
> +	__this_cpu_write(x86_spec_ctrl_current, val);
> +	native_wrmsrl(MSR_IA32_SPEC_CTRL, val);
> +}

Should we also use this to implement update_spec_ctrl() ?

Re: [PATCH v4 1/4] x86/speculation: Add __update_spec_ctrl() helper

[Waiman Long]

On 7/3/23 06:28, Peter Zijlstra wrote:
> On Tue, Jun 27, 2023 at 10:25:51PM -0400, Waiman Long wrote:
>> Add a new __update_spec_ctrl() helper which is a variant of
>> update_spec_ctrl() that can be used in a noinstr function.
>>
>> Suggested-by: Peter Zijlstra <peterz@infradead.org>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   arch/x86/include/asm/nospec-branch.h | 11 ++++++++++-
>>   1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
>> index 55388c9f7601..1d363fcea207 100644
>> --- a/arch/x86/include/asm/nospec-branch.h
>> +++ b/arch/x86/include/asm/nospec-branch.h
>> @@ -9,7 +9,7 @@
>>   
>>   #include <asm/alternative.h>
>>   #include <asm/cpufeatures.h>
>> -#include <asm/msr-index.h>
>> +#include <asm/msr.h>
>>   #include <asm/unwind_hints.h>
>>   #include <asm/percpu.h>
>>   #include <asm/current.h>
>> @@ -488,6 +488,15 @@ DECLARE_PER_CPU(u64, x86_spec_ctrl_current);
>>   extern void update_spec_ctrl_cond(u64 val);
>>   extern u64 spec_ctrl_current(void);
>>   
>> +/*
>> + * This can be used in noinstr function.
>> + */
>> +static __always_inline void __update_spec_ctrl(u64 val)
>> +{
>> +	__this_cpu_write(x86_spec_ctrl_current, val);
>> +	native_wrmsrl(MSR_IA32_SPEC_CTRL, val);
>> +}
> Should we also use this to implement update_spec_ctrl() ?

I also thought about that. However, I am a bit worry about losing some 
additional check done in this_cpu_write() and the paravirt support in 
wrmsrl(). I have no problem making update_spec_ctrl() an instrument-able 
wrapper on __update_spec_ctrl() if you think these are not valid concerns.

Cheers,
Longman

[PATCH v4 2/4] x86/idle: Disable IBRS when cpu is offline

[Waiman Long]

Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle")
disables IBRS when the CPU enters long idle. However, when a CPU
becomes offline, the IBRS bit is still set when X86_FEATURE_KERNEL_IBRS
is enabled. That will impact the performance of a sibling CPU. Mitigate
this performance impact by clearing all the mitigation bits in SPEC_CTRL
MSR when offline. When the CPU is online again, it will be re-initialized
and so restoring the SPEC_CTRL value isn't needed.

Add a comment to say that native_play_dead() is a __noreturn function,
but it can't be marked as such to avoid confusion about the missing
MSR restoration code.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 arch/x86/kernel/smpboot.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index ed2d51960a7d..24ad1ad926a3 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -87,6 +87,7 @@
 #include <asm/hw_irq.h>
 #include <asm/stackprotector.h>
 #include <asm/sev.h>
+#include <asm/nospec-branch.h>
 
 /* representing HT siblings of each logical CPU */
 DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_map);
@@ -1735,8 +1736,15 @@ void __noreturn hlt_play_dead(void)
 		native_halt();
 }
 
+/*
+ * native_play_dead() is essentially a __noreturn function, but it can't
+ * be marked as such as the compiler may complain about it.
+ */
 void native_play_dead(void)
 {
+	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS))
+		__update_spec_ctrl(0);
+
 	play_dead_common();
 	tboot_shutdown(TB_SHUTDOWN_WFS);
 
-- 
2.31.1

[PATCH v4 3/4] intel_idle: Use __update_spec_ctrl() in intel_idle_ibrs()

[Waiman Long]

When intel_idle_ibrs() is called, it modifies the SPEC_CTRL MSR to 0
in order disable IBRS. However, the new MSR value isn't reflected in
x86_spec_ctrl_current which is at odd with the other code that keep track
of its state in that percpu variable.  Use the new __update_spec_ctrl()
to have the x86_spec_ctrl_current percpu value properly updated.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 drivers/idle/intel_idle.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
index 34201d7ef33e..e32ff09051af 100644
--- a/drivers/idle/intel_idle.c
+++ b/drivers/idle/intel_idle.c
@@ -182,12 +182,12 @@ static __cpuidle int intel_idle_ibrs(struct cpuidle_device *dev,
 	int ret;
 
 	if (smt_active)
-		native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
+		__update_spec_ctrl(0);
 
 	ret = __intel_idle(dev, drv, index);
 
 	if (smt_active)
-		native_wrmsrl(MSR_IA32_SPEC_CTRL, spec_ctrl);
+		__update_spec_ctrl(spec_ctrl);
 
 	return ret;
 }
-- 
2.31.1

[PATCH v4 4/4] intel_idle: Add ibrs_off module parameter to force disable IBRS

[Waiman Long]

Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle")
disables IBRS when the cstate is 6 or lower. However, there are
some use cases where a customer may want to use max_cstate=1 to
lower latency. Such use cases will suffer from the performance
degradation caused by the enabling of IBRS in the sibling idle thread.
Add a "ibrs_off" module parameter to force disable IBRS and the
CPUIDLE_FLAG_IRQ_ENABLE flag if set.

In the case of a Skylake server with max_cstate=1, this new ibrs_off
option will likely increase the IRQ response latency as IRQ will now
be disabled.

When running SPECjbb2015 with cstates set to C1 on a Skylake system.

First test when the kernel is booted with: "intel_idle.ibrs_off"
  max-jOPS = 117828, critical-jOPS = 66047

Then retest when the kernel is booted without the "intel_idle.ibrs_off"
added.
  max-jOPS = 116408, critical-jOPS = 58958

That means booting with "intel_idle.ibrs_off" improves performance by:
  max-jOPS:   1.2%, which could be considered noise range.
  critical-jOPS: 12%, which is definitely a solid improvement.

The admin-guide/pm/intel_idle.rst file is updated to add a description
about the new "ibrs_off" module parameter.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++-
 drivers/idle/intel_idle.c                   | 14 ++++++++++++--
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/pm/intel_idle.rst b/Documentation/admin-guide/pm/intel_idle.rst
index b799a43da62e..8604e6d1fe2c 100644
--- a/Documentation/admin-guide/pm/intel_idle.rst
+++ b/Documentation/admin-guide/pm/intel_idle.rst
@@ -170,7 +170,7 @@ and ``idle=nomwait``.  If any of them is present in the kernel command line, the
 ``MWAIT`` instruction is not allowed to be used, so the initialization of
 ``intel_idle`` will fail.
 
-Apart from that there are four module parameters recognized by ``intel_idle``
+Apart from that there are five module parameters recognized by ``intel_idle``
 itself that can be set via the kernel command line (they cannot be updated via
 sysfs, so that is the only way to change their values).
 
@@ -216,6 +216,21 @@ are ignored).
 The idle states disabled this way can be enabled (on a per-CPU basis) from user
 space via ``sysfs``.
 
+The ``ibrs_off`` module parameter is a boolean flag (default to false). It is
+used to control if IBRS (Indirect Branch Restricted Speculation) should be
+turned off, if set, when the CPU enters an idle state.  This flag will not
+affect CPUs that are using Enhanced IBRS which can remain on with little
+performance impact.
+
+For some CPUs, IBRS will be selected as mitigation for Spectre v2 and Retbleed
+security vulnerabilities by default.  Leaving the IBRS mode on while idling may
+have a performance impact on its sibling CPU.  The IBRS mode will be turned off
+by default when the CPU enters into a deep idle state, but not in some
+shallower ones.  Setting the ``ibrs_off`` module parameter will force the IBRS
+mode to off when the CPU is in any one of the available idle states.  This may
+help performance of a sibling CPU at the expense of a slightly higher wakeup
+latency for the idle CPU.
+
 
 .. _intel-idle-core-and-package-idle-states:
 
diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
index e32ff09051af..14874579b396 100644
--- a/drivers/idle/intel_idle.c
+++ b/drivers/idle/intel_idle.c
@@ -69,6 +69,7 @@ static int max_cstate = CPUIDLE_STATE_MAX - 1;
 static unsigned int disabled_states_mask __read_mostly;
 static unsigned int preferred_states_mask __read_mostly;
 static bool force_irq_on __read_mostly;
+static bool ibrs_off __read_mostly;
 
 static struct cpuidle_device __percpu *intel_idle_cpuidle_devices;
 
@@ -1919,12 +1920,15 @@ static void state_update_enter_method(struct cpuidle_state *state, int cstate)
 	}
 
 	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) &&
-			   state->flags & CPUIDLE_FLAG_IBRS) {
+			((state->flags & CPUIDLE_FLAG_IBRS) || ibrs_off)) {
 		/*
 		 * IBRS mitigation requires that C-states are entered
 		 * with interrupts disabled.
 		 */
-		WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
+		if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
+			state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
+		else
+			WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
 		state->enter = intel_idle_ibrs;
 		return;
 	}
@@ -2346,3 +2350,9 @@ MODULE_PARM_DESC(preferred_cstates, "Mask of preferred idle states");
  * 'CPUIDLE_FLAG_INIT_XSTATE' and 'CPUIDLE_FLAG_IBRS' flags.
  */
 module_param(force_irq_on, bool, 0444);
+/*
+ * Force the disabling of IBRS when X86_FEATURE_KERNEL_IBRS is on and
+ * CPUIDLE_FLAG_IRQ_ENABLE isn't set.
+ */
+module_param(ibrs_off, bool, 0444);
+MODULE_PARM_DESC(ibrs_off, "Disable IBRS when idle");
-- 
2.31.1

Re: [PATCH v4 4/4] intel_idle: Add ibrs_off module parameter to force disable IBRS

[Randy Dunlap]

Hi,

On 6/27/23 19:25, Waiman Long wrote:
>  Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++-
>  drivers/idle/intel_idle.c                   | 14 ++++++++++++--
>  2 files changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/pm/intel_idle.rst b/Documentation/admin-guide/pm/intel_idle.rst
> index b799a43da62e..8604e6d1fe2c 100644
> --- a/Documentation/admin-guide/pm/intel_idle.rst
> +++ b/Documentation/admin-guide/pm/intel_idle.rst

You may take or leave these comments as you choose.

> @@ -216,6 +216,21 @@ are ignored).
>  The idle states disabled this way can be enabled (on a per-CPU basis) from user
>  space via ``sysfs``.
>  
> +The ``ibrs_off`` module parameter is a boolean flag (default to false). It is

                                                       (default false).
or
                                                       (defaults to false).

Then I think it reads better if the next sentence begins with: "If set,"
instead of having it in the middle of the sentence.

> +used to control if IBRS (Indirect Branch Restricted Speculation) should be
> +turned off, if set, when the CPU enters an idle state.  This flag will not
> +affect CPUs that are using Enhanced IBRS which can remain on with little
> +performance impact.
> +
> +For some CPUs, IBRS will be selected as mitigation for Spectre v2 and Retbleed
> +security vulnerabilities by default.  Leaving the IBRS mode on while idling may
> +have a performance impact on its sibling CPU.  The IBRS mode will be turned off
> +by default when the CPU enters into a deep idle state, but not in some
> +shallower ones.  Setting the ``ibrs_off`` module parameter will force the IBRS
> +mode to off when the CPU is in any one of the available idle states.  This may
> +help performance of a sibling CPU at the expense of a slightly higher wakeup
> +latency for the idle CPU.

thanks.
-- 
~Randy

Re: [PATCH v4 4/4] intel_idle: Add ibrs_off module parameter to force disable IBRS

[Waiman Long]

On 6/27/23 22:58, Randy Dunlap wrote:
> Hi,
>
> On 6/27/23 19:25, Waiman Long wrote:
>>   Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++-
>>   drivers/idle/intel_idle.c                   | 14 ++++++++++++--
>>   2 files changed, 28 insertions(+), 3 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/pm/intel_idle.rst b/Documentation/admin-guide/pm/intel_idle.rst
>> index b799a43da62e..8604e6d1fe2c 100644
>> --- a/Documentation/admin-guide/pm/intel_idle.rst
>> +++ b/Documentation/admin-guide/pm/intel_idle.rst
> You may take or leave these comments as you choose.
>
>> @@ -216,6 +216,21 @@ are ignored).
>>   The idle states disabled this way can be enabled (on a per-CPU basis) from user
>>   space via ``sysfs``.
>>   
>> +The ``ibrs_off`` module parameter is a boolean flag (default to false). It is
>                                                         (default false).
> or
>                                                         (defaults to false).
>
> Then I think it reads better if the next sentence begins with: "If set,"
> instead of having it in the middle of the sentence.
>
>> +used to control if IBRS (Indirect Branch Restricted Speculation) should be
>> +turned off, if set, when the CPU enters an idle state.  This flag will not
>> +affect CPUs that are using Enhanced IBRS which can remain on with little
>> +performance impact.
>> +
>> +For some CPUs, IBRS will be selected as mitigation for Spectre v2 and Retbleed
>> +security vulnerabilities by default.  Leaving the IBRS mode on while idling may
>> +have a performance impact on its sibling CPU.  The IBRS mode will be turned off
>> +by default when the CPU enters into a deep idle state, but not in some
>> +shallower ones.  Setting the ``ibrs_off`` module parameter will force the IBRS
>> +mode to off when the CPU is in any one of the available idle states.  This may
>> +help performance of a sibling CPU at the expense of a slightly higher wakeup
>> +latency for the idle CPU.
> thanks.

Thanks for the suggestion. I will make the change if I have to revise 
the patch again for any reason.

Cheers,
Longman

Re: [PATCH v4 4/4] intel_idle: Add ibrs_off module parameter to force disable IBRS

[Peter Zijlstra]

On Tue, Jun 27, 2023 at 10:25:54PM -0400, Waiman Long wrote:

> @@ -69,6 +69,7 @@ static int max_cstate = CPUIDLE_STATE_MAX - 1;
>  static unsigned int disabled_states_mask __read_mostly;
>  static unsigned int preferred_states_mask __read_mostly;
>  static bool force_irq_on __read_mostly;
> +static bool ibrs_off __read_mostly;
>  
>  static struct cpuidle_device __percpu *intel_idle_cpuidle_devices;
>  
> @@ -1919,12 +1920,15 @@ static void state_update_enter_method(struct cpuidle_state *state, int cstate)
>  	}
>  
>  	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) &&
> -			   state->flags & CPUIDLE_FLAG_IBRS) {
> +			((state->flags & CPUIDLE_FLAG_IBRS) || ibrs_off)) {
>  		/*
>  		 * IBRS mitigation requires that C-states are entered
>  		 * with interrupts disabled.
>  		 */
> -		WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
> +		if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
> +			state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
> +		else
> +			WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);

If you're respinning this, you can leave out the else and avoid the
indent on the WARN:

+		if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
+			state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
		WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);

Same effect, simpler code and all that.

>  		state->enter = intel_idle_ibrs;
>  		return;
>  	}

Re: [PATCH v4 4/4] intel_idle: Add ibrs_off module parameter to force disable IBRS

[Waiman Long]

On 7/3/23 06:32, Peter Zijlstra wrote:
> On Tue, Jun 27, 2023 at 10:25:54PM -0400, Waiman Long wrote:
>
>> @@ -69,6 +69,7 @@ static int max_cstate = CPUIDLE_STATE_MAX - 1;
>>   static unsigned int disabled_states_mask __read_mostly;
>>   static unsigned int preferred_states_mask __read_mostly;
>>   static bool force_irq_on __read_mostly;
>> +static bool ibrs_off __read_mostly;
>>   
>>   static struct cpuidle_device __percpu *intel_idle_cpuidle_devices;
>>   
>> @@ -1919,12 +1920,15 @@ static void state_update_enter_method(struct cpuidle_state *state, int cstate)
>>   	}
>>   
>>   	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) &&
>> -			   state->flags & CPUIDLE_FLAG_IBRS) {
>> +			((state->flags & CPUIDLE_FLAG_IBRS) || ibrs_off)) {
>>   		/*
>>   		 * IBRS mitigation requires that C-states are entered
>>   		 * with interrupts disabled.
>>   		 */
>> -		WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
>> +		if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
>> +			state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
>> +		else
>> +			WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
> If you're respinning this, you can leave out the else and avoid the
> indent on the WARN:
>
> +		if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
> +			state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
> 		WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
>
> Same effect, simpler code and all that.

That is true. I can certainly respin that as there is another suggested 
doc change that is pending.

Cheers,
Longman

Re: [PATCH v4 0/4] x86/speculation: Disable IBRS when idle

[Rafael J. Wysocki]

On Wed, Jun 28, 2023 at 4:27 AM Waiman Long <longman@redhat.com> wrote:
>
>  v4:
>   - Add a new __update_spec_ctrl() helper in patch 1.
>   - Rebased to the latest linux kernel.
>
>  v3:
>   - Drop patches 1 ("x86/speculation: Provide a debugfs file to dump
>     SPEC_CTRL MSRs") and 5 ("x86/idle: Disable IBRS entering mwait idle
>     and enable it on wakeup") for now.
>   - Drop the MSR restoration code in ("x86/idle: Disable IBRS when cpu
>     is offline") as native_play_dead() does not return.
>   - For patch ("intel_idle: Add ibrs_off module parameter to force
>     disable IBRS"), change the name from "no_ibrs" to "ibrs_off" and
>     document the new parameter in intel_idle.rst.
>
> For Intel processors that need to turn on IBRS to protect against
> Spectre v2 and Retbleed, the IBRS bit in the SPEC_CTRL MSR affects
> the performance of the whole core even if only one thread is turning
> it on when running in the kernel. For user space heavy applications,
> the performance impact of occasionally turning IBRS on during syscalls
> shouldn't be significant. Unfortunately, that is not the case when the
> sibling thread is idling in the kernel. In that case, the performance
> impact can be significant.
>
> When DPDK is running on an isolated CPU thread processing network packets
> in user space while its sibling thread is idle. The performance of the
> busy DPDK thread with IBRS on and off in the sibling idle thread are:
>
>                                 IBRS on         IBRS off
>                                 -------         --------
>   packets/second:                  7.8M           10.4M
>   avg tsc cycles/packet:         282.26          209.86
>
> This is a 25% performance degradation. The test system is a Intel Xeon
> 4114 CPU @ 2.20GHz.
>
> Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle")
> disables IBRS when the CPU enters long idle (C6 or below). However, there
> are existing users out there who have set "intel_idle.max_cstate=1"
> to decrease latency. Those users won't be able to benefit from this
> commit. This patch series extends this commit by providing a new
> "intel_idle.ibrs_off" module parameter to force disable IBRS even when
> "intel_idle.max_cstate=1" at the expense of increased IRQ response
> latency. It also includes a commit to allow the disabling of IBRS when
> a CPU becomes offline.
>
>
> Waiman Long (4):
>   x86/speculation: Add __update_spec_ctrl() helper
>   x86/idle: Disable IBRS when cpu is offline
>   intel_idle: Use __update_spec_ctrl() in intel_idle_ibrs()
>   intel_idle: Add ibrs_off module parameter to force disable IBRS

x86 maintainers, if you want to take care of this series, please feel
free to add

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

to all of the patches in it.

Thanks!