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    SubjectRe: [PATCH 1/3] KVM: SVM: Unconditionally sync GPRs to GHCB on VMRUN of SEV-ES guest
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    On 1/22/21 5:50 PM, Sean Christopherson wrote:
    > Drop the per-GPR dirty checks when synchronizing GPRs to the GHCB, the
    > GRPs' dirty bits are set from time zero and never cleared, i.e. will

    Ah, missed that, bad assumption on my part.

    > always be seen as dirty. The obvious alternative would be to clear
    > the dirty bits when appropriate, but removing the dirty checks is
    > desirable as it allows reverting GPR dirty+available tracking, which
    > adds overhead to all flavors of x86 VMs.
    >
    > Note, unconditionally writing the GPRs in the GHCB is tacitly allowed
    > by the GHCB spec, which allows the hypervisor (or guest) to provide
    > unnecessary info; it's the guest's responsibility to consume only what
    > it needs (the hypervisor is untrusted after all).
    >
    > The guest and hypervisor can supply additional state if desired but
    > must not rely on that additional state being provided.

    Yes, that's true.

    I'm ok with removing the tracking if that's desired. Otherwise, we can add
    a vcpu->arch.regs_dirty = 0 in sev_es_sync_from_ghcb().

    Thanks,
    Tom

    >
    > Cc: Brijesh Singh <brijesh.singh@amd.com>
    > Cc: Tom Lendacky <thomas.lendacky@amd.com>
    > Fixes: 291bd20d5d88 ("KVM: SVM: Add initial support for a VMGEXIT VMEXIT")
    > Signed-off-by: Sean Christopherson <seanjc@google.com>
    > ---
    > arch/x86/kvm/svm/sev.c | 15 ++++++---------
    > 1 file changed, 6 insertions(+), 9 deletions(-)
    >
    > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
    > index c8ffdbc81709..ac652bc476ae 100644
    > --- a/arch/x86/kvm/svm/sev.c
    > +++ b/arch/x86/kvm/svm/sev.c
    > @@ -1415,16 +1415,13 @@ static void sev_es_sync_to_ghcb(struct vcpu_svm *svm)
    > * to be returned:
    > * GPRs RAX, RBX, RCX, RDX
    > *
    > - * Copy their values to the GHCB if they are dirty.
    > + * Copy their values, even if they may not have been written during the
    > + * VM-Exit. It's the guest's responsibility to not consume random data.
    > */
    > - if (kvm_register_is_dirty(vcpu, VCPU_REGS_RAX))
    > - ghcb_set_rax(ghcb, vcpu->arch.regs[VCPU_REGS_RAX]);
    > - if (kvm_register_is_dirty(vcpu, VCPU_REGS_RBX))
    > - ghcb_set_rbx(ghcb, vcpu->arch.regs[VCPU_REGS_RBX]);
    > - if (kvm_register_is_dirty(vcpu, VCPU_REGS_RCX))
    > - ghcb_set_rcx(ghcb, vcpu->arch.regs[VCPU_REGS_RCX]);
    > - if (kvm_register_is_dirty(vcpu, VCPU_REGS_RDX))
    > - ghcb_set_rdx(ghcb, vcpu->arch.regs[VCPU_REGS_RDX]);
    > + ghcb_set_rax(ghcb, vcpu->arch.regs[VCPU_REGS_RAX]);
    > + ghcb_set_rbx(ghcb, vcpu->arch.regs[VCPU_REGS_RBX]);
    > + ghcb_set_rcx(ghcb, vcpu->arch.regs[VCPU_REGS_RCX]);
    > + ghcb_set_rdx(ghcb, vcpu->arch.regs[VCPU_REGS_RDX]);
    > }
    >
    > static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
    >

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