Re: [Xen-devel] [PATCH v9 04/38] x86/CPU/AMD: Add the Secure Memory Encryption CPU feature

[Brian Gerst <brgerst@xxxxxxxxx>]

On Mon, Jul 10, 2017 at 3:41 PM, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
> On 7/8/2017 7:50 AM, Brian Gerst wrote:
>>
>> On Fri, Jul 7, 2017 at 9:38 AM, Tom Lendacky <thomas.lendacky@xxxxxxx>
>> wrote:
>>>
>>> Update the CPU features to include identifying and reporting on the
>>> Secure Memory Encryption (SME) feature.  SME is identified by CPUID
>>> 0x8000001f, but requires BIOS support to enable it (set bit 23 of
>>> MSR_K8_SYSCFG).  Only show the SME feature as available if reported by
>>> CPUID and enabled by BIOS.
>>>
>>> Reviewed-by: Borislav Petkov <bp@xxxxxxx>
>>> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
>>> ---
>>>   arch/x86/include/asm/cpufeatures.h |    1 +
>>>   arch/x86/include/asm/msr-index.h   |    2 ++
>>>   arch/x86/kernel/cpu/amd.c          |   13 +++++++++++++
>>>   arch/x86/kernel/cpu/scattered.c    |    1 +
>>>   4 files changed, 17 insertions(+)
>>>
>>> diff --git a/arch/x86/include/asm/cpufeatures.h
>>> b/arch/x86/include/asm/cpufeatures.h
>>> index 2701e5f..2b692df 100644
>>> --- a/arch/x86/include/asm/cpufeatures.h
>>> +++ b/arch/x86/include/asm/cpufeatures.h
>>> @@ -196,6 +196,7 @@
>>>
>>>   #define X86_FEATURE_HW_PSTATE  ( 7*32+ 8) /* AMD HW-PState */
>>>   #define X86_FEATURE_PROC_FEEDBACK ( 7*32+ 9) /* AMD
>>> ProcFeedbackInterface */
>>> +#define X86_FEATURE_SME                ( 7*32+10) /* AMD Secure Memory
>>> Encryption */
>>
>>
>> Given that this feature is available only in long mode, this should be
>> added to disabled-features.h as disabled for 32-bit builds.
>
>
> I can add that.  If the series needs a re-spin then I'll include this
> change in the series, otherwise I can send a follow-on patch to handle
> the feature for 32-bit builds if that works.
>
>
>>
>>>   #define X86_FEATURE_INTEL_PPIN ( 7*32+14) /* Intel Processor Inventory
>>> Number */
>>>   #define X86_FEATURE_INTEL_PT   ( 7*32+15) /* Intel Processor Trace */
>>> diff --git a/arch/x86/include/asm/msr-index.h
>>> b/arch/x86/include/asm/msr-index.h
>>> index 18b1623..460ac01 100644
>>> --- a/arch/x86/include/asm/msr-index.h
>>> +++ b/arch/x86/include/asm/msr-index.h
>>> @@ -352,6 +352,8 @@
>>>   #define MSR_K8_TOP_MEM1                        0xc001001a
>>>   #define MSR_K8_TOP_MEM2                        0xc001001d
>>>   #define MSR_K8_SYSCFG                  0xc0010010
>>> +#define MSR_K8_SYSCFG_MEM_ENCRYPT_BIT  23
>>> +#define MSR_K8_SYSCFG_MEM_ENCRYPT
>>> BIT_ULL(MSR_K8_SYSCFG_MEM_ENCRYPT_BIT)
>>>   #define MSR_K8_INT_PENDING_MSG         0xc0010055
>>>   /* C1E active bits in int pending message */
>>>   #define K8_INTP_C1E_ACTIVE_MASK                0x18000000
>>> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
>>> index bb5abe8..c47ceee 100644
>>> --- a/arch/x86/kernel/cpu/amd.c
>>> +++ b/arch/x86/kernel/cpu/amd.c
>>> @@ -611,6 +611,19 @@ static void early_init_amd(struct cpuinfo_x86 *c)
>>>           */
>>>          if (cpu_has_amd_erratum(c, amd_erratum_400))
>>>                  set_cpu_bug(c, X86_BUG_AMD_E400);
>>> +
>>> +       /*
>>> +        * BIOS support is required for SME. If BIOS has not enabled SME
>>> +        * then don't advertise the feature (set in scattered.c)
>>> +        */
>>> +       if (cpu_has(c, X86_FEATURE_SME)) {
>>> +               u64 msr;
>>> +
>>> +               /* Check if SME is enabled */
>>> +               rdmsrl(MSR_K8_SYSCFG, msr);
>>> +               if (!(msr & MSR_K8_SYSCFG_MEM_ENCRYPT))
>>> +                       clear_cpu_cap(c, X86_FEATURE_SME);
>>> +       }
>>
>>
>> This should be conditional on CONFIG_X86_64.
>
>
> If I make the scattered feature support conditional on CONFIG_X86_64
> (based on comment below) then cpu_has() will always be false unless
> CONFIG_X86_64 is enabled. So this won't need to be wrapped by the
> #ifdef.

If you change it to use cpu_feature_enabled(), gcc will see that it is
disabled and eliminate the dead code at compile time.

>>
>>>   }
>>>
>>>   static void init_amd_k8(struct cpuinfo_x86 *c)
>>> diff --git a/arch/x86/kernel/cpu/scattered.c
>>> b/arch/x86/kernel/cpu/scattered.c
>>> index 23c2350..05459ad 100644
>>> --- a/arch/x86/kernel/cpu/scattered.c
>>> +++ b/arch/x86/kernel/cpu/scattered.c
>>> @@ -31,6 +31,7 @@ struct cpuid_bit {
>>>          { 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_SME,              CPUID_EAX,  0, 0x8000001f, 0 },
>>
>>
>> This should also be conditional.  We don't want to set this feature on
>> 32-bit, even if the processor has support.
>
>
> Can do.  See comment above about re-spin vs. follow-on patch.
>
> Thanks,
> Tom

A followup patch will be OK if there is no code that will get confused
by the SME bit being present but not active.

--
Brian Gerst

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