| Message ID | 20241022100812.4955-1-jgross@suse.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | kvm/x86: simplify kvm_mmu_do_page_fault() a little bit | expand |
On Tue, Oct 22, 2024, Juergen Gross wrote: > Testing whether to call kvm_tdp_page_fault() or > vcpu->arch.mmu->page_fault() doesn't make sense, as kvm_tdp_page_fault() > is selected only if vcpu->arch.mmu->page_fault == kvm_tdp_page_fault. It does when retpolines are enabled and significantly inflate the cost of the indirect call. This is a hot path in various scenarios, but KVM can't use static_call() to avoid the retpoline due to mmu->page_fault being a property of the current vCPU. Only kvm_tdp_page_fault() is special cased because all other mmu->page_fault targets are slow-ish and/or we don't care terribly about their performance. > Signed-off-by: Juergen Gross <jgross@suse.com> > --- > arch/x86/kvm/mmu/mmu_internal.h | 5 +---- > 1 file changed, 1 insertion(+), 4 deletions(-) > > diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h > index c98827840e07..6eae54aa1160 100644 > --- a/arch/x86/kvm/mmu/mmu_internal.h > +++ b/arch/x86/kvm/mmu/mmu_internal.h > @@ -322,10 +322,7 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, > fault.slot = kvm_vcpu_gfn_to_memslot(vcpu, fault.gfn); > } > > - if (IS_ENABLED(CONFIG_MITIGATION_RETPOLINE) && fault.is_tdp) > - r = kvm_tdp_page_fault(vcpu, &fault); > - else > - r = vcpu->arch.mmu->page_fault(vcpu, &fault); > + r = vcpu->arch.mmu->page_fault(vcpu, &fault); > > /* > * Not sure what's happening, but punt to userspace and hope that > -- > 2.43.0 >
On 22.10.24 19:01, Sean Christopherson wrote: > On Tue, Oct 22, 2024, Juergen Gross wrote: >> Testing whether to call kvm_tdp_page_fault() or >> vcpu->arch.mmu->page_fault() doesn't make sense, as kvm_tdp_page_fault() >> is selected only if vcpu->arch.mmu->page_fault == kvm_tdp_page_fault. > > It does when retpolines are enabled and significantly inflate the cost of the > indirect call. This is a hot path in various scenarios, but KVM can't use > static_call() to avoid the retpoline due to mmu->page_fault being a property of > the current vCPU. Only kvm_tdp_page_fault() is special cased because all other > mmu->page_fault targets are slow-ish and/or we don't care terribly about their > performance. Fair enough. :-) I'll modify the patch to add a comment in this regard in order to avoid similar simplification attempts in the future. Juergen
diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h index c98827840e07..6eae54aa1160 100644 --- a/arch/x86/kvm/mmu/mmu_internal.h +++ b/arch/x86/kvm/mmu/mmu_internal.h @@ -322,10 +322,7 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, fault.slot = kvm_vcpu_gfn_to_memslot(vcpu, fault.gfn); } - if (IS_ENABLED(CONFIG_MITIGATION_RETPOLINE) && fault.is_tdp) - r = kvm_tdp_page_fault(vcpu, &fault); - else - r = vcpu->arch.mmu->page_fault(vcpu, &fault); + r = vcpu->arch.mmu->page_fault(vcpu, &fault); /* * Not sure what's happening, but punt to userspace and hope that
Testing whether to call kvm_tdp_page_fault() or vcpu->arch.mmu->page_fault() doesn't make sense, as kvm_tdp_page_fault() is selected only if vcpu->arch.mmu->page_fault == kvm_tdp_page_fault. Signed-off-by: Juergen Gross <jgross@suse.com> --- arch/x86/kvm/mmu/mmu_internal.h | 5 +---- 1 file changed, 1 insertion(+), 4 deletions(-)