Messages in this thread | | | Date | Thu, 12 Jan 2023 16:21:28 +1100 | Subject | Re: [PATCH kernel v2 1/3] x86/amd: Cache values in percpu variables | From | Alexey Kardashevskiy <> |
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On 11/1/23 03:05, Borislav Petkov wrote: > On Fri, Dec 09, 2022 at 03:38:02PM +1100, Alexey Kardashevskiy wrote: > > Make that Subject: > > "x86/amd: Cache debug register values in percpu variables" > > to make it less generic and more specific as to what you're doing. > >> Reading DR[0-3]_ADDR_MASK MSRs takes about 250 cycles which is going to >> be noticeable with the AMD KVM SEV-ES DebugSwap feature enabled. >> KVM is going to store host's DR[0-3] and DR[0-3]_ADDR_MASK before >> switching to a guest; the hardware is going to swap these on VMRUN >> and VMEXIT. >> >> Store MSR values passsed to set_dr_addr_mask() in percpu values > > s/values/variables/ > > Unknown word [passsed] in commit message. > > Use a spellchecker pls. > >> (when changed) and return them via new amd_get_dr_addr_mask(). >> The gain here is about 10x. > > 10x when reading percpu vars vs MSR reads? > > Oh well. > >> As amd_set_dr_addr_mask() uses the array too, change the @dr type to >> unsigned to avoid checking for <0. > > I feel ya but that function will warn once, return 0 when the @dr number is > outta bounds and that 0 will still get used as an address mask.
"that function" is set_dr_addr_mask() (btw should I rename it to start with amd_? the commit log uses the wrong&longer name) which does not return a mask, amd_get_dr_addr_mask() does.
> I think you really wanna return negative on error and the caller should not > continue in that case.
If it is out of bounds, it won't neither set or get. And these 2 helpers are used only by the kernel and the callers pass 0..3 and nothing else. BUG_ON() would do too, for example.
>> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c >> index c75d75b9f11a..9ac5a19f89b9 100644 >> --- a/arch/x86/kernel/cpu/amd.c >> +++ b/arch/x86/kernel/cpu/amd.c >> @@ -1158,24 +1158,41 @@ static bool cpu_has_amd_erratum(struct cpuinfo_x86 *cpu, const int *erratum) >> return false; >> } >> >> -void set_dr_addr_mask(unsigned long mask, int dr) >> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long[4], amd_dr_addr_mask); > > static > >> + >> +static unsigned int amd_msr_dr_addr_masks[] = { >> + MSR_F16H_DR0_ADDR_MASK, >> + MSR_F16H_DR1_ADDR_MASK - 1 + 1, > > - 1 + 1 ? > > Why? > > Because of the DR0 and then DR1 being in a different MSR range?
Yup.
> > Who cares, make it simple: > > MSR_F16H_DR0_ADDR_MASK, > MSR_F16H_DR1_ADDR_MASK, > MSR_F16H_DR1_ADDR_MASK + 1, > MSR_F16H_DR1_ADDR_MASK + 2 > > and add a comment if you want to denote the non-contiguous range but meh.
imho having 1,2,3 in the code eliminates the need in a comment and produces the exact same end result. But since nobody cares, I'll do it the shorter way with just +1 and +2.
> >> + MSR_F16H_DR1_ADDR_MASK - 1 + 2, >> + MSR_F16H_DR1_ADDR_MASK - 1 + 3 >> +}; >> + >> +void set_dr_addr_mask(unsigned long mask, unsigned int dr) >> { >> - if (!boot_cpu_has(X86_FEATURE_BPEXT)) >> + if (!cpu_feature_enabled(X86_FEATURE_BPEXT)) >> return; >> >> - switch (dr) { >> - case 0: >> - wrmsr(MSR_F16H_DR0_ADDR_MASK, mask, 0); >> - break; >> - case 1: >> - case 2: >> - case 3: >> - wrmsr(MSR_F16H_DR1_ADDR_MASK - 1 + dr, mask, 0); >> - break; >> - default: >> - break; >> - } >> + if (WARN_ON_ONCE(dr >= ARRAY_SIZE(amd_msr_dr_addr_masks))) >> + return; >> + >> + if (per_cpu(amd_dr_addr_mask, smp_processor_id())[dr] == mask) > > Do that at function entry: > > int cpu = smp_processor_id(); > > and use cpu here.
Sure. Out of curiosity - why?^w^w^w^w^ it reduced the vmlinux size by 48 bytes, nice.
Thanks for the review!
> >> + return; >> + >> + wrmsr(amd_msr_dr_addr_masks[dr], mask, 0); >> + per_cpu(amd_dr_addr_mask, smp_processor_id())[dr] = mask; >> +} > > Thx. >
-- Alexey
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