linux-kernel - Re: [PATCHV2 1/3] KVM: x86: introduce kvm_mmu_pte

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Date:   Thu, 21 Oct 2021 17:09:29 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     David Matlack <dmatlack@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCHV2 1/3] KVM: x86: introduce kvm_mmu_pte_prefetch structure

On (21/10/21 12:28), Sergey Senozhatsky wrote:
> > 
> > We are using TDP. And somehow I never see (literally never) async PFs.
> > It's always either hva_to_pfn_fast() or hva_to_pfn_slow() or
> > __direct_map() from tdp_page_fault().
> 
> Hmm, and tdp_page_fault()->fast_page_fault() always fails on
> !is_access_allowed(error_code, new_spte), it never handles the faults.
> And I see some ->mmu_lock contention:
> 
> 	spin_lock(&vcpu->kvm->mmu_lock);
> 	__direct_map();
> 	spin_unlock(&vcpu->kvm->mmu_lock);
> 
> So it might be that we setup guest memory wrongly and never get
> advantages of TPD and fast page faults?

No, never mind, that's probably expected and ->mmu_lock contention is not
severe.