Re: [PATCH 2/2] KVM: VMX: Skip GUEST_CR3 VMREAD+VMWRITE if the VMCS is up-to-date

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Fri, Sep 27, 2019 at 02:11:27PM +0200, Vitaly Kuznetsov wrote:
> 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()).

Part of me wants to keep it painful to discourage one-off checks, but
yeah, a helper would be nice.

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