Re: [PATCH v2 14/18] KVM: x86/mmu: avoid indirect call for get_cr3

Sean Christopherson
2022-02-18T20:30:23Z

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.