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

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Date:   Mon, 22 Jan 2018 08:31:52 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>,
        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 12:01 PM, Andrew Cooper wrote:
> 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@...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.
> 
> 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.

The behavior is "read as written", so the bit will be updated even though
the support for the bit is not present.

Thanks,
Tom

> 
> Thanks,
> 
> ~Andrew
>