linux-kernel - Re: [PATCH v14 17/22] KVM: SEV: Avoid WBINVD for HVA-based MMU notifications for SNP

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Message-ID: <ZimhDMwAif-KIxNs@google.com>
Date: Wed, 24 Apr 2024 17:17:16 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Michael Roth <michael.roth@....com>
Cc: kvm@...r.kernel.org, linux-coco@...ts.linux.dev, linux-mm@...ck.org, 
	linux-crypto@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org, 
	tglx@...utronix.de, mingo@...hat.com, jroedel@...e.de, 
	thomas.lendacky@....com, hpa@...or.com, ardb@...nel.org, pbonzini@...hat.com, 
	vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org, 
	dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com, 
	peterz@...radead.org, srinivas.pandruvada@...ux.intel.com, 
	rientjes@...gle.com, dovmurik@...ux.ibm.com, tobin@....com, bp@...en8.de, 
	vbabka@...e.cz, kirill@...temov.name, ak@...ux.intel.com, tony.luck@...el.com, 
	sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com, 
	jarkko@...nel.org, ashish.kalra@....com, nikunj.dadhania@....com, 
	pankaj.gupta@....com, liam.merwick@...cle.com
Subject: Re: [PATCH v14 17/22] KVM: SEV: Avoid WBINVD for HVA-based MMU
 notifications for SNP

On Sun, Apr 21, 2024, Michael Roth wrote:
> From: Ashish Kalra <ashish.kalra@....com>
> 
> With SNP/guest_memfd, private/encrypted memory should not be mappable,
> and MMU notifications for HVA-mapped memory will only be relevant to
> unencrypted guest memory. Therefore, the rationale behind issuing a
> wbinvd_on_all_cpus() in sev_guest_memory_reclaimed() should not apply
> for SNP guests and can be ignored.
> 
> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
> Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
> [mdr: Add some clarifications in commit]
> Signed-off-by: Michael Roth <michael.roth@....com>
> ---
>  arch/x86/kvm/svm/sev.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 243369e302f4..cf00a811aca5 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3042,7 +3042,14 @@ static void sev_flush_encrypted_page(struct kvm_vcpu *vcpu, void *va)
>  
>  void sev_guest_memory_reclaimed(struct kvm *kvm)
>  {
> -	if (!sev_guest(kvm))
> +	/*
> +	 * With SNP+gmem, private/encrypted memory should be

Not should, *is*.

> +	 * unreachable via the hva-based mmu notifiers. Additionally,
> +	 * for shared->private translations, H/W coherency will ensure
> +	 * first guest access to the page would clear out any existing
> +	 * dirty copies of that cacheline.

I don't see how this second part is relevant.

> +	 */
> +	if (!sev_guest(kvm) || sev_snp_guest(kvm))
>  		return;
>  
>  	wbinvd_on_all_cpus();
> -- 
> 2.25.1
>