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SubjectRe: [PATCH 2/2] KVM: VMX: Skip GUEST_CR3 VMREAD+VMWRITE if the VMCS is up-to-date
Date
Sean Christopherson <sean.j.christopherson@intel.com> writes:

> Skip the VMWRITE to update GUEST_CR3 if CR3 is not available, i.e. has
> not been read from the VMCS since the last VM-Enter. If vcpu->arch.cr3
> is stale, kvm_read_cr3(vcpu) will refresh vcpu->arch.cr3 from the VMCS,
> meaning KVM will do a VMREAD and then VMWRITE the value it just pulled
> from the VMCS.
>
> Note, this is a purely theoretical change, no instances of skipping
> the VMREAD+VMWRITE have been observed with this change.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
> arch/x86/kvm/vmx/vmx.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index b530950a9c2b..6de09f60edf3 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -3003,10 +3003,12 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>
> if (is_guest_mode(vcpu))
> skip_cr3 = true;
> - else if (enable_unrestricted_guest || is_paging(vcpu))
> - guest_cr3 = kvm_read_cr3(vcpu);
> - else
> + else if (!enable_unrestricted_guest && !is_paging(vcpu))
> guest_cr3 = to_kvm_vmx(kvm)->ept_identity_map_addr;
> + else if (test_bit(VCPU_EXREG_CR3, (ulong *)&vcpu->arch.regs_avail))

Nit: with 'test_bit(,(ulong *)&vcpu->arch.regs_avail)' spreading more and
more I'd suggest creating an inline in kvm_cache_regs.h
(e.g. kvm_vcpu_reg_avail()).

> + guest_cr3 = vcpu->arch.cr3;
> + else
> + skip_cr3 = true; /* vmcs01.GUEST_CR3 is up-to-date. */
> ept_load_pdptrs(vcpu);
> }

--
Vitaly

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