[PATCH v3 0/3] riscv: Per-thread envcfg CSR support

[PATCH v3 0/3] riscv: Per-thread envcfg CSR support

[Samuel Holland]

This series (or equivalent) is a prerequisite for both user-mode pointer
masking and CFI support, as both of those are per-thread features and
are controlled by fields in the envcfg CSR. These patches are based on
v1 of the pointer masking series[1], with significant input from both
Deepak and Andrew.

[1]: https://lore.kernel.org/linux-riscv/20240319215915.832127-6-samuel.holland@sifive.com/

Changes in v3:
 - Rebase on riscv/for-next
 - Drop use of __initdata due to conflicts with cpufeature.c refactoring

Changes in v2:
 - Rebase on riscv/for-next

Samuel Holland (3):
  riscv: Enable cbo.zero only when all harts support Zicboz
  riscv: Add support for per-thread envcfg CSR values
  riscv: Call riscv_user_isa_enable() only on the boot hart

 arch/riscv/include/asm/cpufeature.h |  2 +-
 arch/riscv/include/asm/processor.h  |  1 +
 arch/riscv/include/asm/switch_to.h  |  8 ++++++++
 arch/riscv/kernel/cpufeature.c      | 11 ++++++++---
 arch/riscv/kernel/smpboot.c         |  2 --
 arch/riscv/kernel/suspend.c         |  4 ++--
 6 files changed, 20 insertions(+), 8 deletions(-)

-- 
2.45.1

[PATCH v3 1/3] riscv: Enable cbo.zero only when all harts support Zicboz

[Samuel Holland]

Currently, we enable cbo.zero for usermode on each hart that supports
the Zicboz extension. This means that the [ms]envcfg CSR value may
differ between harts. Other features, such as pointer masking and CFI,
require setting [ms]envcfg bits on a per-thread basis. The combination
of these two adds quite some complexity and overhead to context
switching, as we would need to maintain two separate masks for the
per-hart and per-thread bits. Andrew Jones, who originally added Zicboz
support, writes[1][2]:

  I've approached Zicboz the same way I would approach all
  extensions, which is to be per-hart. I'm not currently aware of
  a platform that is / will be composed of harts where some have
  Zicboz and others don't, but there's nothing stopping a platform
  like that from being built.

  So, how about we add code that confirms Zicboz is on all harts.
  If any hart does not have it, then we complain loudly and disable
  it on all the other harts. If it was just a hardware description
  bug, then it'll get fixed. If there's actually a platform which
  doesn't have Zicboz on all harts, then, when the issue is reported,
  we can decide to not support it, support it with defconfig, or
  support it under a Kconfig guard which must be enabled by the user.

Let's follow his suggested solution and require the extension to be
available on all harts, so the envcfg CSR value does not need to change
when a thread migrates between harts. Since we are doing this for all
extensions with fields in envcfg, the CSR itself only needs to be saved/
restored when it is present on all harts.

This should not be a regression as no known hardware has asymmetric
Zicboz support, but if anyone reports seeing the warning, we will
re-evaluate our solution.

Link: https://lore.kernel.org/linux-riscv/20240322-168f191eeb8479b2ea169a5e@orel/ [1]
Link: https://lore.kernel.org/linux-riscv/20240323-28943722feb57a41fb0ff488@orel/ [2]
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Deepak Gupta <debug@rivosinc.com>
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
---

Changes in v3:
 - Rebase on riscv/for-next

 arch/riscv/kernel/cpufeature.c | 7 ++++++-
 arch/riscv/kernel/suspend.c    | 4 ++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 0366dc3baf33..4efaf12b470e 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -27,6 +27,8 @@
 
 #define NUM_ALPHA_EXTS ('z' - 'a' + 1)
 
+static bool any_cpu_has_zicboz;
+
 unsigned long elf_hwcap __read_mostly;
 
 /* Host ISA bitmap */
@@ -97,6 +99,7 @@ static int riscv_ext_zicboz_validate(const struct riscv_isa_ext_data *data,
 		pr_err("Zicboz disabled as cboz-block-size present, but is not a power-of-2\n");
 		return -EINVAL;
 	}
+	any_cpu_has_zicboz = true;
 	return 0;
 }
 
@@ -874,8 +877,10 @@ unsigned long riscv_get_elf_hwcap(void)
 
 void riscv_user_isa_enable(void)
 {
-	if (riscv_cpu_has_extension_unlikely(smp_processor_id(), RISCV_ISA_EXT_ZICBOZ))
+	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICBOZ))
 		csr_set(CSR_ENVCFG, ENVCFG_CBZE);
+	else if (any_cpu_has_zicboz)
+		pr_warn_once("Zicboz disabled as it is unavailable on some harts\n");
 }
 
 #ifdef CONFIG_RISCV_ALTERNATIVE
diff --git a/arch/riscv/kernel/suspend.c b/arch/riscv/kernel/suspend.c
index c8cec0cc5833..9a8a0dc035b2 100644
--- a/arch/riscv/kernel/suspend.c
+++ b/arch/riscv/kernel/suspend.c
@@ -14,7 +14,7 @@
 
 void suspend_save_csrs(struct suspend_context *context)
 {
-	if (riscv_cpu_has_extension_unlikely(smp_processor_id(), RISCV_ISA_EXT_XLINUXENVCFG))
+	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_XLINUXENVCFG))
 		context->envcfg = csr_read(CSR_ENVCFG);
 	context->tvec = csr_read(CSR_TVEC);
 	context->ie = csr_read(CSR_IE);
@@ -37,7 +37,7 @@ void suspend_save_csrs(struct suspend_context *context)
 void suspend_restore_csrs(struct suspend_context *context)
 {
 	csr_write(CSR_SCRATCH, 0);
-	if (riscv_cpu_has_extension_unlikely(smp_processor_id(), RISCV_ISA_EXT_XLINUXENVCFG))
+	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_XLINUXENVCFG))
 		csr_write(CSR_ENVCFG, context->envcfg);
 	csr_write(CSR_TVEC, context->tvec);
 	csr_write(CSR_IE, context->ie);
-- 
2.45.1

[PATCH v3 2/3] riscv: Add support for per-thread envcfg CSR values

[Samuel Holland]

Some bits in the [ms]envcfg CSR, such as the CFI state and pointer
masking mode, need to be controlled on a per-thread basis. Support this
by keeping a copy of the CSR value in struct thread_struct and writing
it during context switches. It is safe to discard the old CSR value
during the context switch because the CSR is modified only by software,
so the CSR will remain in sync with the copy in thread_struct.

Use ALTERNATIVE directly instead of riscv_has_extension_unlikely() to
minimize branchiness in the context switching code.

Since thread_struct is copied during fork(), setting the value for the
init task sets the default value for all other threads.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Deepak Gupta <debug@rivosinc.com>
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
---

(no changes since v1)

 arch/riscv/include/asm/processor.h | 1 +
 arch/riscv/include/asm/switch_to.h | 8 ++++++++
 arch/riscv/kernel/cpufeature.c     | 2 +-
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index 8702b8721a27..586e4ab701c4 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -124,6 +124,7 @@ struct thread_struct {
 	unsigned long s[12];	/* s[0]: frame pointer */
 	struct __riscv_d_ext_state fstate;
 	unsigned long bad_cause;
+	unsigned long envcfg;
 	u32 riscv_v_flags;
 	u32 vstate_ctrl;
 	struct __riscv_v_ext_state vstate;
diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
index 7594df37cc9f..9685cd85e57c 100644
--- a/arch/riscv/include/asm/switch_to.h
+++ b/arch/riscv/include/asm/switch_to.h
@@ -70,6 +70,13 @@ static __always_inline bool has_fpu(void) { return false; }
 #define __switch_to_fpu(__prev, __next) do { } while (0)
 #endif
 
+static inline void __switch_to_envcfg(struct task_struct *next)
+{
+	asm volatile (ALTERNATIVE("nop", "csrw " __stringify(CSR_ENVCFG) ", %0",
+				  0, RISCV_ISA_EXT_XLINUXENVCFG, 1)
+			:: "r" (next->thread.envcfg) : "memory");
+}
+
 extern struct task_struct *__switch_to(struct task_struct *,
 				       struct task_struct *);
 
@@ -103,6 +110,7 @@ do {							\
 		__switch_to_vector(__prev, __next);	\
 	if (switch_to_should_flush_icache(__next))	\
 		local_flush_icache_all();		\
+	__switch_to_envcfg(__next);			\
 	((last) = __switch_to(__prev, __next));		\
 } while (0)
 
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 4efaf12b470e..43fdae953379 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -878,7 +878,7 @@ unsigned long riscv_get_elf_hwcap(void)
 void riscv_user_isa_enable(void)
 {
 	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICBOZ))
-		csr_set(CSR_ENVCFG, ENVCFG_CBZE);
+		current->thread.envcfg |= ENVCFG_CBZE;
 	else if (any_cpu_has_zicboz)
 		pr_warn_once("Zicboz disabled as it is unavailable on some harts\n");
 }
-- 
2.45.1

[PATCH v3 3/3] riscv: Call riscv_user_isa_enable() only on the boot hart

[Samuel Holland]

Now that the [ms]envcfg CSR value is maintained per thread, not per
hart, riscv_user_isa_enable() only needs to be called once during boot,
to set the value for the init task. This also allows it to be marked as
__init.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Deepak Gupta <debug@rivosinc.com>
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
---

Changes in v3:
 - Drop use of __initdata due to conflicts with cpufeature.c refactoring

Changes in v2:
 - Rebase on riscv/for-next

 arch/riscv/include/asm/cpufeature.h | 2 +-
 arch/riscv/kernel/cpufeature.c      | 4 ++--
 arch/riscv/kernel/smpboot.c         | 2 --
 3 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
index 000796c2d0b1..7670792760b6 100644
--- a/arch/riscv/include/asm/cpufeature.h
+++ b/arch/riscv/include/asm/cpufeature.h
@@ -31,7 +31,7 @@ DECLARE_PER_CPU(struct riscv_cpuinfo, riscv_cpuinfo);
 /* Per-cpu ISA extensions. */
 extern struct riscv_isainfo hart_isa[NR_CPUS];
 
-void riscv_user_isa_enable(void);
+void __init riscv_user_isa_enable(void);
 
 #if defined(CONFIG_RISCV_MISALIGNED)
 bool check_unaligned_access_emulated_all_cpus(void);
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 43fdae953379..517035356107 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -875,12 +875,12 @@ unsigned long riscv_get_elf_hwcap(void)
 	return hwcap;
 }
 
-void riscv_user_isa_enable(void)
+void __init riscv_user_isa_enable(void)
 {
 	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICBOZ))
 		current->thread.envcfg |= ENVCFG_CBZE;
 	else if (any_cpu_has_zicboz)
-		pr_warn_once("Zicboz disabled as it is unavailable on some harts\n");
+		pr_warn("Zicboz disabled as it is unavailable on some harts\n");
 }
 
 #ifdef CONFIG_RISCV_ALTERNATIVE
diff --git a/arch/riscv/kernel/smpboot.c b/arch/riscv/kernel/smpboot.c
index 19baf0d574d3..0646f79e0a02 100644
--- a/arch/riscv/kernel/smpboot.c
+++ b/arch/riscv/kernel/smpboot.c
@@ -235,8 +235,6 @@ asmlinkage __visible void smp_callin(void)
 	numa_add_cpu(curr_cpuid);
 	set_cpu_online(curr_cpuid, true);
 
-	riscv_user_isa_enable();
-
 	/*
 	 * Remote cache and TLB flushes are ignored while the CPU is offline,
 	 * so flush them both right now just in case.
-- 
2.45.1

Re: [PATCH v3 1/3] riscv: Enable cbo.zero only when all harts support Zicboz

[Charlie Jenkins]

On Wed, Jul 17, 2024 at 05:47:54PM -0700, Samuel Holland wrote:
> Currently, we enable cbo.zero for usermode on each hart that supports
> the Zicboz extension. This means that the [ms]envcfg CSR value may
> differ between harts. Other features, such as pointer masking and CFI,
> require setting [ms]envcfg bits on a per-thread basis. The combination
> of these two adds quite some complexity and overhead to context
> switching, as we would need to maintain two separate masks for the
> per-hart and per-thread bits. Andrew Jones, who originally added Zicboz
> support, writes[1][2]:
> 
>   I've approached Zicboz the same way I would approach all
>   extensions, which is to be per-hart. I'm not currently aware of
>   a platform that is / will be composed of harts where some have
>   Zicboz and others don't, but there's nothing stopping a platform
>   like that from being built.
> 
>   So, how about we add code that confirms Zicboz is on all harts.
>   If any hart does not have it, then we complain loudly and disable
>   it on all the other harts. If it was just a hardware description
>   bug, then it'll get fixed. If there's actually a platform which
>   doesn't have Zicboz on all harts, then, when the issue is reported,
>   we can decide to not support it, support it with defconfig, or
>   support it under a Kconfig guard which must be enabled by the user.
> 
> Let's follow his suggested solution and require the extension to be
> available on all harts, so the envcfg CSR value does not need to change
> when a thread migrates between harts. Since we are doing this for all
> extensions with fields in envcfg, the CSR itself only needs to be saved/
> restored when it is present on all harts.
> 
> This should not be a regression as no known hardware has asymmetric
> Zicboz support, but if anyone reports seeing the warning, we will
> re-evaluate our solution.
> 
> Link: https://lore.kernel.org/linux-riscv/20240322-168f191eeb8479b2ea169a5e@orel/ [1]
> Link: https://lore.kernel.org/linux-riscv/20240323-28943722feb57a41fb0ff488@orel/ [2]
> Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
> Reviewed-by: Deepak Gupta <debug@rivosinc.com>
> Signed-off-by: Samuel Holland <samuel.holland@sifive.com>

Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Charlie Jenkins <charlie@rivosinc.com>

> ---
> 
> Changes in v3:
>  - Rebase on riscv/for-next
> 
>  arch/riscv/kernel/cpufeature.c | 7 ++++++-
>  arch/riscv/kernel/suspend.c    | 4 ++--
>  2 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 0366dc3baf33..4efaf12b470e 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -27,6 +27,8 @@
>  
>  #define NUM_ALPHA_EXTS ('z' - 'a' + 1)
>  
> +static bool any_cpu_has_zicboz;
> +
>  unsigned long elf_hwcap __read_mostly;
>  
>  /* Host ISA bitmap */
> @@ -97,6 +99,7 @@ static int riscv_ext_zicboz_validate(const struct riscv_isa_ext_data *data,
>  		pr_err("Zicboz disabled as cboz-block-size present, but is not a power-of-2\n");
>  		return -EINVAL;
>  	}
> +	any_cpu_has_zicboz = true;
>  	return 0;
>  }
>  
> @@ -874,8 +877,10 @@ unsigned long riscv_get_elf_hwcap(void)
>  
>  void riscv_user_isa_enable(void)
>  {
> -	if (riscv_cpu_has_extension_unlikely(smp_processor_id(), RISCV_ISA_EXT_ZICBOZ))
> +	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICBOZ))
>  		csr_set(CSR_ENVCFG, ENVCFG_CBZE);
> +	else if (any_cpu_has_zicboz)
> +		pr_warn_once("Zicboz disabled as it is unavailable on some harts\n");
>  }
>  
>  #ifdef CONFIG_RISCV_ALTERNATIVE
> diff --git a/arch/riscv/kernel/suspend.c b/arch/riscv/kernel/suspend.c
> index c8cec0cc5833..9a8a0dc035b2 100644
> --- a/arch/riscv/kernel/suspend.c
> +++ b/arch/riscv/kernel/suspend.c
> @@ -14,7 +14,7 @@
>  
>  void suspend_save_csrs(struct suspend_context *context)
>  {
> -	if (riscv_cpu_has_extension_unlikely(smp_processor_id(), RISCV_ISA_EXT_XLINUXENVCFG))
> +	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_XLINUXENVCFG))
>  		context->envcfg = csr_read(CSR_ENVCFG);
>  	context->tvec = csr_read(CSR_TVEC);
>  	context->ie = csr_read(CSR_IE);
> @@ -37,7 +37,7 @@ void suspend_save_csrs(struct suspend_context *context)
>  void suspend_restore_csrs(struct suspend_context *context)
>  {
>  	csr_write(CSR_SCRATCH, 0);
> -	if (riscv_cpu_has_extension_unlikely(smp_processor_id(), RISCV_ISA_EXT_XLINUXENVCFG))
> +	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_XLINUXENVCFG))
>  		csr_write(CSR_ENVCFG, context->envcfg);
>  	csr_write(CSR_TVEC, context->tvec);
>  	csr_write(CSR_IE, context->ie);
> -- 
> 2.45.1
> 

Re: [PATCH v3 2/3] riscv: Add support for per-thread envcfg CSR values

[Charlie Jenkins]

On Wed, Jul 17, 2024 at 05:47:55PM -0700, Samuel Holland wrote:
> Some bits in the [ms]envcfg CSR, such as the CFI state and pointer
> masking mode, need to be controlled on a per-thread basis. Support this
> by keeping a copy of the CSR value in struct thread_struct and writing
> it during context switches. It is safe to discard the old CSR value
> during the context switch because the CSR is modified only by software,
> so the CSR will remain in sync with the copy in thread_struct.
> 
> Use ALTERNATIVE directly instead of riscv_has_extension_unlikely() to
> minimize branchiness in the context switching code.
> 
> Since thread_struct is copied during fork(), setting the value for the
> init task sets the default value for all other threads.
> 
> Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
> Reviewed-by: Deepak Gupta <debug@rivosinc.com>
> Signed-off-by: Samuel Holland <samuel.holland@sifive.com>

Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>

> ---
> 
> (no changes since v1)
> 
>  arch/riscv/include/asm/processor.h | 1 +
>  arch/riscv/include/asm/switch_to.h | 8 ++++++++
>  arch/riscv/kernel/cpufeature.c     | 2 +-
>  3 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> index 8702b8721a27..586e4ab701c4 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -124,6 +124,7 @@ struct thread_struct {
>  	unsigned long s[12];	/* s[0]: frame pointer */
>  	struct __riscv_d_ext_state fstate;
>  	unsigned long bad_cause;
> +	unsigned long envcfg;
>  	u32 riscv_v_flags;
>  	u32 vstate_ctrl;
>  	struct __riscv_v_ext_state vstate;
> diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
> index 7594df37cc9f..9685cd85e57c 100644
> --- a/arch/riscv/include/asm/switch_to.h
> +++ b/arch/riscv/include/asm/switch_to.h
> @@ -70,6 +70,13 @@ static __always_inline bool has_fpu(void) { return false; }
>  #define __switch_to_fpu(__prev, __next) do { } while (0)
>  #endif
>  
> +static inline void __switch_to_envcfg(struct task_struct *next)
> +{
> +	asm volatile (ALTERNATIVE("nop", "csrw " __stringify(CSR_ENVCFG) ", %0",
> +				  0, RISCV_ISA_EXT_XLINUXENVCFG, 1)
> +			:: "r" (next->thread.envcfg) : "memory");
> +}
> +
>  extern struct task_struct *__switch_to(struct task_struct *,
>  				       struct task_struct *);
>  
> @@ -103,6 +110,7 @@ do {							\
>  		__switch_to_vector(__prev, __next);	\
>  	if (switch_to_should_flush_icache(__next))	\
>  		local_flush_icache_all();		\
> +	__switch_to_envcfg(__next);			\
>  	((last) = __switch_to(__prev, __next));		\
>  } while (0)
>  
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 4efaf12b470e..43fdae953379 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -878,7 +878,7 @@ unsigned long riscv_get_elf_hwcap(void)
>  void riscv_user_isa_enable(void)
>  {
>  	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICBOZ))
> -		csr_set(CSR_ENVCFG, ENVCFG_CBZE);
> +		current->thread.envcfg |= ENVCFG_CBZE;
>  	else if (any_cpu_has_zicboz)
>  		pr_warn_once("Zicboz disabled as it is unavailable on some harts\n");
>  }
> -- 
> 2.45.1
>