Re: [PATCH 24/24] kvm: x86/mmu: Allow parallel page faults for the TDP MMU

Sean Christopherson
2021-01-21T02:09:39Z

On Tue, Jan 12, 2021, Ben Gardon wrote:
> Make the last few changes necessary to enable the TDP MMU to handle page
> faults in parallel while holding the mmu_lock in read mode.
>
> Reviewed-by: Peter Feiner <pfeiner@google.com>
>
> Signed-off-by: Ben Gardon <bgardon@google.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 280d7cd6f94b..fa111ceb67d4 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3724,7 +3724,12 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> return r;
>
> r = RET_PF_RETRY;
> - kvm_mmu_lock(vcpu->kvm);
> +
> + if (is_tdp_mmu_root(vcpu->kvm, vcpu->arch.mmu->root_hpa))

Off topic, what do you think about rewriting is_tdp_mmu_root() to be both more
performant and self-documenting as to when is_tdp_mmu_root() !=
kvm->arch.tdp_mmu_enabled? E.g. key off is_guest_mode() and then do a thorough
audit/check when CONFIG_KVM_MMU_AUDIT=y?

#ifdef CONFIG_KVM_MMU_AUDIT
bool is_tdp_mmu_root(struct kvm *kvm, hpa_t hpa)
{
struct kvm_mmu_page *sp;

if (!kvm->arch.tdp_mmu_enabled)
return false;
if (WARN_ON(!VALID_PAGE(hpa)))
return false;

sp = to_shadow_page(hpa);
if (WARN_ON(!sp))
return false;

return sp->tdp_mmu_page && sp->root_count;
}
#endif

bool is_tdp_mmu(struct kvm_vcpu *vcpu)
{
bool is_tdp_mmu = kvm->arch.tdp_mmu_enabled && !is_guest_mode(vcpu);

#ifdef CONFIG_KVM_MMU_AUDIT
WARN_ON(is_tdp_mmu != is_tdp_mmu_root(vcpu->kvm, vcpu->arch.mmu->root_hpa));
#endif
return is_tdp_mmu;
}

> + kvm_mmu_lock_shared(vcpu->kvm);
> + else
> + kvm_mmu_lock(vcpu->kvm);
> +
> if (mmu_notifier_retry(vcpu->kvm, mmu_seq))
> goto out_unlock;
> r = make_mmu_pages_available(vcpu);
> @@ -3739,7 +3744,10 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> prefault, is_tdp);
>
> out_unlock:
> - kvm_mmu_unlock(vcpu->kvm);
> + if (is_tdp_mmu_root(vcpu->kvm, vcpu->arch.mmu->root_hpa))
> + kvm_mmu_unlock_shared(vcpu->kvm);
> + else
> + kvm_mmu_unlock(vcpu->kvm);
> kvm_release_pfn_clean(pfn);
> return r;
> }
> --
> 2.30.0.284.gd98b1dd5eaa7-goog
>