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

From: Charlie Jenkins
Date: Thu Sep 12 2024 - 18:57:07 EST


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@xxxxxxxxxxxxxxxx>
> Reviewed-by: Deepak Gupta <debug@xxxxxxxxxxxx>
> Signed-off-by: Samuel Holland <samuel.holland@xxxxxxxxxx>

Reviewed-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>

> ---
>
> (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
>