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

Paolo Bonzini
2022-02-19T10:03:57Z

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@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.

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