Re: [Patch v2 3/4] x86/sev: Add guest support for Enhanced SMT Protection

[Pratik R. Sampat]

On 10/9/26 3:18 PM, Tom Lendacky wrote:
> On 10/6/26 11:55, Pratik R. Sampat wrote:
>> Enhanced SMT Protection requires every VMSA to carry the vCPU's identity
>> and the mask of SMT siblings it may co-run with. The guest builds the
>> VMSA itself when bringing up an AP, so populate both fields there, before
>> the page is turned into a VMSA and while it is still writable.
>>
>> Fully set the sibling mask so that all threads of the same guest trust
>> each other.
>>
>> Signed-off-by: Pratik R. Sampat <prsampat@xxxxxxx>
> 
> Minor comment below, but otherwise:
> 
> Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> 
>> ---
>> v1..v2:
>> * Add ESMTP to SNP_FEATURES_IMPL and SNP_FEATURES_IMPL_REQ  
>> ---
>>  arch/x86/boot/compressed/sev.c   |  6 ++++--
>>  arch/x86/coco/sev/core.c         | 13 +++++++++++++
>>  arch/x86/include/asm/msr-index.h |  4 +++-
>>  3 files changed, 20 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
>> index c6512f2ea31e..4e0fa5a5f70b 100644
>> --- a/arch/x86/boot/compressed/sev.c
>> +++ b/arch/x86/boot/compressed/sev.c
>> @@ -188,7 +188,8 @@ bool sev_es_check_ghcb_fault(unsigned long address)
>>  				 MSR_AMD64_SNP_RESERVED_BIT13 |		\
>>  				 MSR_AMD64_SNP_RESERVED_BIT15 |		\
>>  				 MSR_AMD64_SNP_SECURE_AVIC |		\
>> -				 MSR_AMD64_SNP_RESERVED_BITS19_22 |	\
>> +				 MSR_AMD64_SNP_ESMT_PROT |		\
>> +				 MSR_AMD64_SNP_RESERVED_BITS20_22 |	\
>>  				 MSR_AMD64_SNP_RESERVED_MASK)
>>  
>>  #ifdef CONFIG_AMD_SECURE_AVIC
>> @@ -204,7 +205,8 @@ bool sev_es_check_ghcb_fault(unsigned long address)
>>   */
>>  #define SNP_FEATURES_IMPL	(MSR_AMD64_SNP_DEBUG_SWAP |	\
>>  				 MSR_AMD64_SNP_SECURE_TSC |	\
>> -				 SNP_FEATURE_SECURE_AVIC)
>> +				 SNP_FEATURE_SECURE_AVIC |	\
>> +				 MSR_AMD64_SNP_ESMT_PROT)
>>  
>>  u64 snp_get_unsupported_features(u64 status)
>>  {
>> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
>> index cc292d7c6fd1..730cff2700d4 100644
>> --- a/arch/x86/coco/sev/core.c
>> +++ b/arch/x86/coco/sev/core.c
>> @@ -89,6 +89,7 @@ static const char * const sev_status_feat_names[] = {
>>  	[MSR_AMD64_SNP_VMSA_REG_PROT_BIT]	= "VMSARegProt",
>>  	[MSR_AMD64_SNP_SMT_PROT_BIT]		= "SMTProt",
>>  	[MSR_AMD64_SNP_SECURE_AVIC_BIT]		= "SecureAVIC",
>> +	[MSR_AMD64_SNP_ESMT_PROT_BIT]		= "ESMTProt",
>>  	[MSR_AMD64_SNP_IBPB_ON_ENTRY_BIT]	= "IBPBOnEntry",
>>  };
>>  
>> @@ -849,6 +850,18 @@ static int wakeup_cpu_via_vmgexit(u32 apic_id, unsigned long start_ip, unsigned
>>  	vmsa->vmpl		= snp_vmpl;
>>  	vmsa->sev_features	= sev_status >> 2;
>>  
>> +	if (cc_platform_has(CC_ATTR_GUEST_SEV_SNP) &&
>> +	    (sev_status & MSR_AMD64_SNP_ESMT_PROT)) {
> 
> You're using sev_status for ESMT_PROT, might as well use it for SNP, too,
> instead of calling cc_platform_has().
> 

Sure, will do.

Thank you!
--Pratik

> Thanks,
> Tom
> 
>> +		vmsa->vcpu_id = apic_id;
>> +		/*
>> +		 * The mask fully set puts every vCPU in one group, so any two
>> +		 * of them may be co-resident. A legal ESMTP sibling must
>> +		 * also match on ASID, so the sibling of a vCPU in guest mode is
>> +		 * always either another vCPU of this same guest or idle.
>> +		 */
>> +		vmsa->vcpu_sibling_mask = U32_MAX;
>> +	}
>> +
>>  	/* Populate AP's TSC scale/offset to get accurate TSC values. */
>>  	if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC)) {
>>  		vmsa->tsc_scale = snp_tsc_scale;
>> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
>> index fbcb3313e946..6fd26151d146 100644
>> --- a/arch/x86/include/asm/msr-index.h
>> +++ b/arch/x86/include/asm/msr-index.h
>> @@ -745,7 +745,9 @@
>>  #define MSR_AMD64_SNP_SMT_PROT		BIT_ULL(MSR_AMD64_SNP_SMT_PROT_BIT)
>>  #define MSR_AMD64_SNP_SECURE_AVIC_BIT	18
>>  #define MSR_AMD64_SNP_SECURE_AVIC	BIT_ULL(MSR_AMD64_SNP_SECURE_AVIC_BIT)
>> -#define MSR_AMD64_SNP_RESERVED_BITS19_22 GENMASK_ULL(22, 19)
>> +#define MSR_AMD64_SNP_ESMT_PROT_BIT	19
>> +#define MSR_AMD64_SNP_ESMT_PROT	BIT_ULL(MSR_AMD64_SNP_ESMT_PROT_BIT)
>> +#define MSR_AMD64_SNP_RESERVED_BITS20_22 GENMASK_ULL(22, 20)
>>  #define MSR_AMD64_SNP_IBPB_ON_ENTRY_BIT	23
>>  #define MSR_AMD64_SNP_IBPB_ON_ENTRY	BIT_ULL(MSR_AMD64_SNP_IBPB_ON_ENTRY_BIT)
>>  #define MSR_AMD64_SNP_RESV_BIT		24
>