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Date: Thu, 8 Dec 2022 17:11:26 +1100
From: Alexey Kardashevskiy <aik@....com>
To: Borislav Petkov <bp@...en8.de>
Cc: kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
Venu Busireddy <venu.busireddy@...cle.com>,
Tony Luck <tony.luck@...el.com>,
Tom Lendacky <thomas.lendacky@....com>,
Thomas Gleixner <tglx@...utronix.de>,
Sean Christopherson <seanjc@...gle.com>,
Peter Zijlstra <peterz@...radead.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Michael Sterritt <sterritt@...gle.com>,
Michael Roth <michael.roth@....com>,
Mario Limonciello <mario.limonciello@....com>,
Ingo Molnar <mingo@...hat.com>,
Heiko Carstens <hca@...ux.ibm.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Andrew Cooper <andrew.cooper3@...rix.com>,
"Jason A. Donenfeld" <Jason@...c4.com>,
"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH kernel 1/3] x86/amd/dr_addr_mask: Cache values in percpu
variables
On 8/12/22 05:55, Borislav Petkov wrote:
> 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@....com>
>> ---
>> 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;
> }
Not an array, as Sean suggested? Uff...
>
> Thx.
>
--
Alexey
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