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

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Date:   Wed, 20 Jan 2021 16:55:52 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 24/24] kvm: x86/mmu: Allow parallel page faults for the
 TDP MMU

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@...gle.com>
> 
> Signed-off-by: Ben Gardon <bgardon@...gle.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
>