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

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Message-ID: <ce7ea88c-c5d4-154c-37fd-477d517914f3@amd.com>
Date:   Sun, 21 Jan 2018 11:50:20 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     David Woodhouse <dwmw@...zon.co.uk>, arjan@...ux.intel.com,
        tglx@...utronix.de, karahmed@...zon.de, x86@...nel.org,
        linux-kernel@...r.kernel.org, tim.c.chen@...ux.intel.com,
        bp@...en8.de, peterz@...radead.org, pbonzini@...hat.com,
        ak@...ux.intel.com, torvalds@...ux-foundation.org,
        gregkh@...ux-foundation.org
Subject: Re: [PATCH v2 2/8] x86/cpufeatures: Add AMD feature bits for
 Prediction Command

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@...zon.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.

Thanks,
Tom

>  	{ X86_FEATURE_SME,		CPUID_EAX,  0, 0x8000001f, 0 },
>  	{ 0, 0, 0, 0, 0 }
>  };
>