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

[Paolo Bonzini]

On 2/18/22 21:30, Sean Christopherson wrote:
> 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@xxxxxxxxxx>
> > ---
> >   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.

I did it this way to avoid a slightly gratuitous extern function just 
because kvm_mmu_get_guest_pgd and kvm_read_cr3 are both inline.  But at 
least it's not an export since there are no callers in vmx/svm, so it's 
okay to do it as you suggested.

> #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.

No problem.

Paolo