Re: [PATCH 2/2] KVM: SVM: Don't flush cache of SEV-encrypted pages if hardware enforces cache coherency across encryption domains

[Tom Lendacky <thomas.lendacky@amd.com>]

On 8/28/20 7:59 PM, Krish Sadhukhan wrote:
> Some hardware implementations may enforce cache coherency across encryption
> domains. In such cases, it's not required to flush SEV-encrypted pages off
> cache lines.
> 
> Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.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 402dc4234e39..c8ed8a62d5ef 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -384,7 +384,8 @@ static void sev_clflush_pages(struct page *pages[], unsigned long npages)
>   	uint8_t *page_virtual;
>   	unsigned long i;
>   
> -	if (npages == 0 || pages == NULL)
> +	if ((cpuid_eax(SVM_SME_CPUID_FUNC) & (1u << 10)) || npages == 0 ||

Thanks for the patch. This should really be added as an X86_FEATURE bit, 
and then check that feature here, as opposed to calling CPUID every time. 
Also, there are other places in the kernel that this may be relevant, are 
you investigating those areas, also (e.g. set_memory_encrypted() / 
set_memory_decrypted())?

Thanks,
Tom

> +	    pages == NULL)
>   		return;
>   
>   	for (i = 0; i < npages; i++) {
>