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    SubjectRe: [PATCH v2 14/18] KVM: x86/mmu: avoid indirect call for get_cr3
    On Thu, Feb 17, 2022, Paolo Bonzini wrote:
    > Most of the time, calls to get_guest_pgd result in calling
    > kvm_read_cr3 (the exception is only nested TDP). Hardcode
    > the default instead of using the get_cr3 function, avoiding
    > a retpoline if they are enabled.
    >
    > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    > ---
    > arch/x86/kvm/mmu.h | 13 +++++++++++++
    > arch/x86/kvm/mmu/mmu.c | 15 +++++----------
    > arch/x86/kvm/mmu/paging_tmpl.h | 2 +-
    > arch/x86/kvm/x86.c | 2 +-
    > 4 files changed, 20 insertions(+), 12 deletions(-)
    >
    > diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
    > index 1d0c1904d69a..1808d6814ddb 100644
    > --- a/arch/x86/kvm/mmu.h
    > +++ b/arch/x86/kvm/mmu.h
    > @@ -116,6 +116,19 @@ static inline void kvm_mmu_load_pgd(struct kvm_vcpu *vcpu)
    > vcpu->arch.mmu->shadow_root_level);
    > }
    >
    > +static inline gpa_t __kvm_mmu_get_guest_pgd(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu)
    > +{

    I'd prefer to do what we do for page faults. That approach avoids the need for a
    comment to document NULL and avoids a conditional when RETPOLINE is not enabled.

    Might be worth renaming get_cr3 => get_guest_cr3 though.

    #ifdef CONFIG_RETPOLINE
    if (mmu->get_guest_pgd = get_guest_cr3)
    return kvm_read_cr3(vcpu);
    #endif
    return mmu->get_guest_pgd(vcpu);


    > + if (!mmu->get_guest_pgd)
    > + return kvm_read_cr3(vcpu);
    > + else
    > + return mmu->get_guest_pgd(vcpu);
    > +}
    > +
    > +static inline gpa_t kvm_mmu_get_guest_pgd(struct kvm_vcpu *vcpu)
    > +{
    > + return __kvm_mmu_get_guest_pgd(vcpu, vcpu->arch.mmu);

    I'd much prefer we don't provide an @vcpu-only variant and force the caller to
    provide the mmu.

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