Re: [Qemu-devel] [PATCH 1/2] i386: define the AMD 'amd-ssbd' CPUID feature bit

[Tom Lendacky]

On 6/4/2018 3:07 PM, Eduardo Habkost wrote:
> On Fri, Jun 01, 2018 at 11:38:08AM -0400, Konrad Rzeszutek Wilk wrote:
>> AMD future CPUs expose _two_ ways to utilize the Intel equivalant
>> of the Speculative Store Bypass Disable. The first is via
>> the virtualized VIRT_SPEC CTRL MSR (0xC001_011f) and the second
>> is via the SPEC_CTRL MSR (0x48). The document titled:
>> 124441_AMD64_SpeculativeStoreBypassDisable_Whitepaper_final.pdf
>>
>> gives priority of SPEC CTRL MSR over the VIRT SPEC CTRL MSR.
>>
>> A copy of this document is available at
>>       https://bugzilla.kernel.org/show_bug.cgi?id=199889
>>
>> Anyhow, this means that on future AMD CPUs there will be  _two_ ways to
>> deal with SSBD.
> 
> Does anybody know if there are AMD CPUs where virt-ssbd won't
> work and would require amd-ssbd to mitigate vulnerabilities?

The idea behind virt-ssbd was to provide an architectural method for
a guest to do SSBD when amd-ssbd isn't present.  The amd-ssbd feature
will use SPEC_CTRL which is intended to not be intercepted and
will be fast.  The use of virt-ssbd will always be intercepted and
therefore will not be as fast.  So a guest should be presented with
amd-ssbd, if available, in preference to virt-ssbd.

Thanks,
Tom

> 
> Also, do we have kernel arch/x86/kvm/cpuid.c patches, already?
> I prefer to add new CPUID flag names only after the flag name is
> already agreed upon on the kernel side.
> 
> 
>>
>> Signed-off-by: Konrad Rzeszutek Wilk <address@hidden>
>> ---
>>  target/i386/cpu.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
>> index 52d334a..f91990c 100644
>> --- a/target/i386/cpu.c
>> +++ b/target/i386/cpu.c
>> @@ -490,7 +490,7 @@ static FeatureWordInfo feature_word_info[FEATURE_WORDS] 
>> = {
>>              "ibpb", NULL, NULL, NULL,
>>              NULL, NULL, NULL, NULL,
>>              NULL, NULL, NULL, NULL,
>> -            NULL, "virt-ssbd", NULL, NULL,
>> +            "amd-ssbd", "virt-ssbd", NULL, NULL,
>>              NULL, NULL, NULL, NULL,
>>          },
>>          .cpuid_eax = 0x80000008,
>> -- 
>> 1.8.3.1
>>
>>
>