Re: [PATCH kernel 1/3] x86/amd/dr_addr_mask: Cache values in percpu variables

[Borislav Petkov]

On Thu, Dec 01, 2022 at 01:19:46PM +1100, Alexey Kardashevskiy wrote:
> Subject: Re: [PATCH kernel 1/3] x86/amd/dr_addr_mask: Cache values in percpu variables

"x86/amd: " is perfectly fine as a prefix.

> Reading DR[0-3]_ADDR_MASK MSRs takes about 250 cycles which is going to
> be noticeable when the AMD KVM SEV-ES's DebugSwap feature is enabled and

which does what? I.e., a sort of lazy DR regs swapping...

> KVM needs to store these before switching to a guest; the DebugSwitch
> hardware support restores them as type B swap.

I know this is all clear to you but you should explain what type B
register swap is.

> This stores MSR values from set_dr_addr_mask() in percpu values and

s/This stores/Store/

>From Documentation/process/submitting-patches.rst:

 "Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
  instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
  to do frotz", as if you are giving orders to the codebase to change
  its behaviour."

Also, do not talk about what your patch does - that should hopefully be
visible in the diff itself. Rather, talk about *why* you're doing what
you're doing.

> returns them via new get_dr_addr_mask(). The gain here is about 10x.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxx>
> ---
>  arch/x86/include/asm/debugreg.h |  1 +
>  arch/x86/kernel/cpu/amd.c       | 32 ++++++++++++++++++++
>  2 files changed, 33 insertions(+)
> 
> diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
> index cfdf307ddc01..c4324d0205b5 100644
> --- a/arch/x86/include/asm/debugreg.h
> +++ b/arch/x86/include/asm/debugreg.h
> @@ -127,6 +127,7 @@ static __always_inline void local_db_restore(unsigned long dr7)
>  
>  #ifdef CONFIG_CPU_SUP_AMD
>  extern void set_dr_addr_mask(unsigned long mask, int dr);
> +extern unsigned long get_dr_addr_mask(int dr);
>  #else
>  static inline void set_dr_addr_mask(unsigned long mask, int dr) { }
>  #endif
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index c75d75b9f11a..ec7efcef4e14 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -1158,6 +1158,11 @@ static bool cpu_has_amd_erratum(struct cpuinfo_x86 *cpu, const int *erratum)
>  	return false;
>  }
>  
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr0_addr_mask);
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr1_addr_mask);
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr2_addr_mask);
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr3_addr_mask);

This BPEXT thing is AMD-only, right?

I guess those should be called amd_drX_addr_mask where X in [0-3].

Yeah yeah, they are used in AMD-only code - svm* - but still.

>  void set_dr_addr_mask(unsigned long mask, int dr)
>  {
>  	if (!boot_cpu_has(X86_FEATURE_BPEXT))
> @@ -1166,17 +1171,44 @@ void set_dr_addr_mask(unsigned long mask, int dr)
>  	switch (dr) {
>  	case 0:
>  		wrmsr(MSR_F16H_DR0_ADDR_MASK, mask, 0);
> +		per_cpu(dr0_addr_mask, smp_processor_id()) = mask;
>  		break;
>  	case 1:
> +		wrmsr(MSR_F16H_DR1_ADDR_MASK - 1 + dr, mask, 0);
> +		per_cpu(dr1_addr_mask, smp_processor_id()) = mask;
> +		break;
>  	case 2:
> +		wrmsr(MSR_F16H_DR1_ADDR_MASK - 1 + dr, mask, 0);
> +		per_cpu(dr2_addr_mask, smp_processor_id()) = mask;
> +		break;
>  	case 3:
>  		wrmsr(MSR_F16H_DR1_ADDR_MASK - 1 + dr, mask, 0);
> +		per_cpu(dr3_addr_mask, smp_processor_id()) = mask;
>  		break;
>  	default:
>  		break;
>  	}
>  }
>  
> +unsigned long get_dr_addr_mask(int dr)

This function name is too generic for an exported function.

amd_get_dr_addr_mask() I'd say.

> +	if (!boot_cpu_has(X86_FEATURE_BPEXT))

check_for_deprecated_apis: WARNING: arch/x86/kernel/cpu/amd.c:1195: Do not use boot_cpu_has() - use cpu_feature_enabled() instead.

You could fix the above one too, while at it.

> +		return 0;
> +
> +	switch (dr) {
> +	case 0:
> +		return per_cpu(dr0_addr_mask, smp_processor_id());
> +	case 1:
> +		return per_cpu(dr1_addr_mask, smp_processor_id());
> +	case 2:
> +		return per_cpu(dr2_addr_mask, smp_processor_id());
> +	case 3:
> +		return per_cpu(dr3_addr_mask, smp_processor_id());

	default:	
		WARN_ON_ONCE(1);
		break;

Just in case.

And as a matter of fact, make that short and succinct:

        switch (dr) {
        case 0: return per_cpu(dr0_addr_mask, smp_processor_id());
        case 1: return per_cpu(dr1_addr_mask, smp_processor_id());
        case 2: return per_cpu(dr2_addr_mask, smp_processor_id());
        case 3: return per_cpu(dr3_addr_mask, smp_processor_id());
        default: WARN_ON_ONCE(1); break;
        }

Thx.

-- 
Regards/Gruss,
    Boris.

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