linux-kernel - Re: [PATCH v12 18/29] KVM: SEV: Use a VMSA physical address variable for populating VMCB

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Date: Tue, 16 Apr 2024 13:53:24 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Michael Roth <michael.roth@....com>, kvm@...r.kernel.org
Cc: 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, seanjc@...gle.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 v12 18/29] KVM: SEV: Use a VMSA physical address variable
 for populating VMCB

On Sat, Mar 30, 2024 at 10:01 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 3/29/24 23:58, Michael Roth wrote:
> > From: Tom Lendacky<thomas.lendacky@....com>
> >
> > In preparation to support SEV-SNP AP Creation, use a variable that holds
> > the VMSA physical address rather than converting the virtual address.
> > This will allow SEV-SNP AP Creation to set the new physical address that
> > will be used should the vCPU reset path be taken.
> >
> > Signed-off-by: Tom Lendacky<thomas.lendacky@....com>
> > Signed-off-by: Ashish Kalra<ashish.kalra@....com>
> > Signed-off-by: Michael Roth<michael.roth@....com>
> > ---
>
> I'll get back to this one after Easter, but it looks like Sean had some
> objections at https://lore.kernel.org/lkml/ZeCqnq7dLcJI41O9@google.com/.

So IIUC the gist of the solution here would be to replace

   /* Use the new VMSA */
   svm->sev_es.vmsa_pa = pfn_to_hpa(pfn);
   svm->vmcb->control.vmsa_pa = svm->sev_es.vmsa_pa;

with something like

   /* Use the new VMSA */
   __free_page(virt_to_page(svm->sev_es.vmsa));
   svm->sev_es.vmsa = pfn_to_kaddr(pfn);
   svm->vmcb->control.vmsa_pa = __pa(svm->sev_es.vmsa);

and wrap the __free_page() in sev_free_vcpu() with "if
(!svm->sev_es.snp_ap_create)".

This should remove the need for svm->sev_es.vmsa_pa. It is always
equal to svm->vmcb->control.vmsa_pa anyway.

Also, it's possible to remove

   /*
    * gmem pages aren't currently migratable, but if this ever
    * changes then care should be taken to ensure
    * svm->sev_es.vmsa_pa is pinned through some other means.
    */
   kvm_release_pfn_clean(pfn);

if sev_free_vcpu() does

   if (svm->sev_es.snp_ap_create) {
     __free_page(virt_to_page(svm->sev_es.vmsa));
   } else {
     put_page(virt_to_page(svm->sev_es.vmsa));
   }

and while at it, please reverse the polarity of snp_ap_create and
rename it to snp_ap_created.

Paolo