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 14:33:21 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Tom Lendacky <thomas.lendacky@....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 22/01/18 14:31, Tom Lendacky wrote:
> 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.

Fantastic!  Thanks for confirming.

~Andrew