Re: [RFC PATCH 2/4] KVM: SEV: Skip SNP-safe allocation when HvInUseWrAllowed is supported

["Nikunj A. Dadhania" <nikunj@amd.com>]

On 8/26/2025 3:37 PM, Huang, Kai wrote:
> On Mon, 2025-08-25 at 15:20 +0000, Nikunj A Dadhania wrote:
>> When HvInUseWrAllowed (CPUID 8000001F EAX[30]) is supported, the CPU allows
>> hypervisor writes to in-use pages without RMP violations, making the 2MB
>> alignment workaround unnecessary. Check for this capability to avoid the
>> allocation overhead when it's not needed.
> 
> Could you add some text to explain why this is related to PML?

PML works fine without this and the change is not linked to PML. This can go
as a separate fix.

While working on PML which was using the snp_safe_alloc_page(), Tom had suggested
to apply the alignment workaround only on the CPUs without HvInUseWrAllowed.

> 
>>
>> Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
>> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
>> ---
>>  arch/x86/kvm/svm/sev.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>> index 2fbdebf79fbb..c5477efc90b9 100644
>> --- a/arch/x86/kvm/svm/sev.c
>> +++ b/arch/x86/kvm/svm/sev.c
>> @@ -4666,7 +4666,8 @@ struct page *snp_safe_alloc_page_node(int node, gfp_t gfp)
>>  	unsigned long pfn;
>>  	struct page *p;
>>  
>> -	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
>> +	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP) ||
>> +	    cpu_feature_enabled(X86_FEATURE_HV_INUSE_WR_ALLOWED))
>>  		return alloc_pages_node(node, gfp | __GFP_ZERO, 0);
>>  
>>  	/*
Regards,
Nikunj