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.
https://people.kernel.org/tglx/notes-about-netiquette
