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SubjectRe: [PATCH v2 2/8] x86/cpufeatures: Add AMD feature bits for Prediction Command
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On 21/01/18 17:50, Tom Lendacky wrote:
> On 1/21/2018 3:49 AM, David Woodhouse wrote:
>> AMD doesn't implement the Speculation Control MSR that Intel does, but
>> the Prediction Control MSR does exist and is advertised by a separate
>> CPUID bit. Add support for that.
>>
>> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
>> ---
>> arch/x86/include/asm/cpufeatures.h | 1 +
>> arch/x86/kernel/cpu/scattered.c | 1 +
>> 2 files changed, 2 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
>> index 2efb8d4..8c9e5c0 100644
>> --- a/arch/x86/include/asm/cpufeatures.h
>> +++ b/arch/x86/include/asm/cpufeatures.h
>> @@ -207,6 +207,7 @@
>> #define X86_FEATURE_RETPOLINE_AMD ( 7*32+13) /* AMD Retpoline mitigation for Spectre variant 2 */
>> #define X86_FEATURE_INTEL_PPIN ( 7*32+14) /* Intel Processor Inventory Number */
>>
>> +#define X86_FEATURE_AMD_PRED_CMD ( 7*32+17) /* Prediction Command MSR (AMD) */
>> #define X86_FEATURE_MBA ( 7*32+18) /* Memory Bandwidth Allocation */
>> #define X86_FEATURE_RSB_CTXSW ( 7*32+19) /* Fill RSB on context switches */
>>
>> diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c
>> index df11f5d..4eb90b2 100644
>> --- a/arch/x86/kernel/cpu/scattered.c
>> +++ b/arch/x86/kernel/cpu/scattered.c
>> @@ -28,6 +28,7 @@ static const struct cpuid_bit cpuid_bits[] = {
>> { X86_FEATURE_HW_PSTATE, CPUID_EDX, 7, 0x80000007, 0 },
>> { X86_FEATURE_CPB, CPUID_EDX, 9, 0x80000007, 0 },
>> { X86_FEATURE_PROC_FEEDBACK, CPUID_EDX, 11, 0x80000007, 0 },
>> + { X86_FEATURE_AMD_PRED_CMD, CPUID_EBX, 12, 0x80000008, 0 },
> I replied to the previous version, but I'll add it here, too.
>
> This should be moved to the existing 0x80000008/EBX entry rather than have
> it in scattered.
>
> Also, there will be a total of three bits:
> IBPB: 0x80000008 EBX[12]
> IBRS: 0x80000008 EBX[14]
> STIBP: 0x80000008 EBX[15]
>
> Since IBRS and STIBP share the same MSR, if a processor only supports
> STIBP (MSR bit 1), for ease of software implementation the processor
> does not GP fault attempts to write bit 0. In a similar manner, if a
> processor only suppors IBRS (MSR bit 0), the processor does not GP
> fault attempts to write bit 1.

Are you able to comment on the read behaviour after a write which is
ignored?

If the behaviour is "read as written" then virt cases are fine.  If the
"ignore" causes a zero to be read back, then we're still going to need
to intercept and emulate all VM accesses.

Thanks,

~Andrew

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