Re: [PATCH v2 2/8] x86/cpufeatures: Add AMD feature bits for Prediction Command

[Andrew Cooper <andrew.cooper3@citrix.com>]

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