Re: [PATCH kernel v2 1/3] x86/amd: Cache values in percpu variables

Alexey Kardashevskiy
2023-01-12T05:21:54Z

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