linux-kernel - Re: [PATCH v4 1/1] KVM: Introduce KVM_EXIT_SNP_REQ

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Message-ID: <CAAH4kHaCTGxQ_D+KbhJQ+RYL4n5qeG4UnDNu5FK9+3KJLpNw0Q@mail.gmail.com>
Date: Tue, 21 Jan 2025 09:19:08 -0800
From: Dionna Amalie Glaze <dionnaglaze@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Melody Wang <huibo.wang@....com>, kvm@...r.kernel.org, linux-coco@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, roedel@...e.de, 
	Tom Lendacky <thomas.lendacky@....com>, ashish.kalra@....com, liam.merwick@...cle.com, 
	pankaj.gupta@....com, Michael Roth <michael.roth@....com>
Subject: Re: [PATCH v4 1/1] KVM: Introduce KVM_EXIT_SNP_REQ_CERTS for SNP certificate-fetching

On Tue, Jan 21, 2025 at 8:52 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, Jan 21, 2025, Dionna Amalie Glaze wrote:
> > On Mon, Jan 20, 2025 at 1:58 PM Melody Wang <huibo.wang@....com> wrote:
> > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > > index 943bd074a5d3..4896c34ed318 100644
> > > --- a/arch/x86/kvm/svm/sev.c
> > > +++ b/arch/x86/kvm/svm/sev.c
> > > @@ -4064,6 +4064,30 @@ static int snp_handle_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t resp_
> > >         return ret;
> > >  }
> > >
> > > +static int snp_complete_req_certs(struct kvm_vcpu *vcpu)
> > > +{
> > > +       struct vcpu_svm *svm = to_svm(vcpu);
> > > +       struct vmcb_control_area *control = &svm->vmcb->control;
> > > +
> > > +       if (vcpu->run->snp_req_certs.ret) {
> > > +               if (vcpu->run->snp_req_certs.ret == ENOSPC) {
> > > +                       vcpu->arch.regs[VCPU_REGS_RBX] = vcpu->run->snp_req_certs.npages;
> > > +                       ghcb_set_sw_exit_info_2(svm->sev_es.ghcb,
> > > +                                               SNP_GUEST_ERR(SNP_GUEST_VMM_ERR_INVALID_LEN, 0));
> > > +               } else if (vcpu->run->snp_req_certs.ret == EAGAIN) {
> > > +                       ghcb_set_sw_exit_info_2(svm->sev_es.ghcb,
> > > +                                               SNP_GUEST_ERR(SNP_GUEST_VMM_ERR_BUSY, 0));
> >
> > Discussion, not a change request: given that my proposed patch [1] to
> > add rate-limiting for guest messages to the PSP generally was
> > rejected,
>
> For the record, it wasn't rejected outright.  I pointed out flaws in the proposed
> behavior[*], and AFAICT no one ever responded.  If I fully reject something, I
> promise I will make it abundantly clear :-)
>

Okay, well it was a no to the implementation strategy I had chosen and
did not have bandwidth to change. Your suggestion to exit to userspace
is more aligned with the topic of discussion now.

> [*] https://lore.kernel.org/all/Y8rEFpbMV58yJIKy@google.com
>
> > do we think it'd be proper to add a KVM_EXIT_SNP_REQ_MSG or
> > some such for the VMM to decide if the guest should have access to the
> > globally shared resource (PSP) via EAGAIN or 0?
>
> Can you elaborate?  I don't quite understand what you're suggesting.
>

I just mean that instead of only exiting to the VMM on an extended
guest request and this capability enabled, all guest requests exit to
the VMM to make the decision to permit access to the device. EAGAIN
means busy, try again later, and 0 means permit the request. That
allows for implementation-specific throttling policies to be
implemented.

> > [1] https://patchwork.kernel.org/project/kvm/cover/20230119213426.379312-1-dionnaglaze@google.com/
> >
> > > +               } else {
> > > +                       ghcb_set_sw_exit_info_2(svm->sev_es.ghcb,
> > > +                                               SNP_GUEST_ERR(SNP_GUEST_VMM_ERR_GENERIC, 0));
> > > +               }
> > > +
> > > +               return 1; /* resume guest */
> > > +       }
> > > +
> > > +       return snp_handle_guest_req(svm, control->exit_info_1, control->exit_info_2);
> > > +}



-- 
-Dionna Glaze, PhD, CISSP, CCSP (she/her)