linux-kernel - Re: [PATCH v5 4/6] KVM: SVM: Add support to handle AP reset MSR protocol

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Date:   Wed, 20 Oct 2021 18:38:06 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, x86@...nel.org,
        Brijesh Singh <brijesh.singh@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH v5 4/6] KVM: SVM: Add support to handle AP reset MSR
 protocol

On Wed, Oct 20, 2021, Tom Lendacky wrote:
> On 10/20/21 12:40 PM, Sean Christopherson wrote:
> > On Wed, Oct 20, 2021, Joerg Roedel wrote:
> > This can race with the SIPI and effectively corrupt svm->vmcb->control.ghcb_gpa.
> > 
> >          vCPU0                           vCPU1
> >                                          #VMGEXIT(RESET_HOLD)
> >                                          __kvm_vcpu_halt()
> >          INIT
> >          SIPI
> >          sev_vcpu_deliver_sipi_vector()
> >          ghcb_msr_ap_rst_resp(1);
> 
> This isn't possible. vCPU0 doesn't set vCPU1's GHCB value. vCPU1's GHCB
> value is set when vCPU1 handles events in vcpu_enter_guest().

Argh, I was thinking of injecting regular IPIs across vCPUs.  In hindsight it
makes sense that INIT and SIPI are handled on the current vCPU, stuffing all that
state from a different vCPU would be needlessly complex.