From: Maxim Levitsky <mlevitsk@redhat.com>
To: Sean Christopherson <seanjc@google.com>, Ben Gardon <bgardon@google.com>
Cc: kvm <kvm@vger.kernel.org>,
"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<linux-kernel@vger.kernel.org>, Jim Mattson <jmattson@google.com>,
Joerg Roedel <joro@8bytes.org>, Borislav Petkov <bp@alien8.de>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel.org>
Subject: Re: [PATCH v2 8/8] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use
Date: Thu, 29 Jul 2021 17:10:07 +0300 [thread overview]
Message-ID: <7fd945b4845fb436c284d5741057a10dd919a8f6.camel@redhat.com> (raw)
In-Reply-To: <YQBNvLg8WZiKVLBx@google.com>
On Tue, 2021-07-27 at 18:17 +0000, Sean Christopherson wrote:
> On Tue, Jul 27, 2021, Ben Gardon wrote:
> > On Tue, Jul 27, 2021 at 6:06 AM Maxim Levitsky <mlevitsk@redhat.com> wrote:
> > > On Thu, 2021-07-22 at 19:06 +0000, Sean Christopherson wrote:
> > > > The elevated mmu_notifier_count and/or changed mmu_notifier_seq will cause vCPU1
> > > > to bail and resume the guest without fixing the #NPF. After acquiring mmu_lock,
> > > > vCPU1 will see the elevated mmu_notifier_count (if kvm_zap_gfn_range() is about
> > > > to be called, or just finised) and/or a modified mmu_notifier_seq (after the
> > > > count was decremented).
> > > >
> > > > This is why kvm_zap_gfn_range() needs to take mmu_lock for write. If it's allowed
> > > > to run in parallel with the page fault handler, there's no guarantee that the
> > > > correct apic_access_memslot_enabled will be observed.
> > >
> > > I understand now.
> > >
> > > So, Paolo, Ben Gardon, what do you think. Do you think this approach is feasable?
> > > Do you agree to revert the usage of the read lock?
> > >
> > > I will post a new series using this approach very soon, since I already have
> > > msot of the code done.
> > >
> > > Best regards,
> > > Maxim Levitsky
> >
> > From reading through this thread, it seems like switching from read
> > lock to write lock is only necessary for a small range of GFNs, (i.e.
> > the APIC access page) is that correct?
>
> For the APICv case, yes, literally a single GFN (the default APIC base).
>
> > My initial reaction was that switching kvm_zap_gfn_range back to the
> > write lock would be terrible for performance, but given its only two
> > callers, I think it would actually be fine.
>
> And more importantly, the two callers are gated by kvm_arch_has_noncoherent_dma()
> and are very rare flows for the guest (updating MTRRs, toggling CR0.CD).
>
> > If you do that though, you should pass shared=false to
> > kvm_tdp_mmu_zap_gfn_range in that function, so that it knows it's
> > operating with exclusive access to the MMU lock.
>
> Ya, my suggested revert was to drop @shared entirely since kvm_zap_gfn_range() is
> the only caller that passes @shared=true.
>
Just one question:
Should I submit the patches for MMU changes that you described,
and on top of them my AVIC patches?
Should I worry about the new TDP mmu?
Best regards,
Maxim Levitsky
next prev parent reply other threads:[~2021-07-29 14:12 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-13 14:20 [PATCH v2 0/8] My AVIC patch queue Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 1/8] KVM: SVM: svm_set_vintr don't warn if AVIC is active but is about to be deactivated Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 2/8] KVM: SVM: tweak warning about enabled AVIC on nested entry Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 3/8] KVM: SVM: use vmcb01 in svm_refresh_apicv_exec_ctrl Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 4/8] KVM: x86: APICv: drop immediate APICv disablement on current vCPU Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 5/8] KVM: x86: APICv: fix race in kvm_request_apicv_update on SVM Maxim Levitsky
2021-07-26 22:34 ` Paolo Bonzini
2021-07-27 13:22 ` Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 6/8] KVM: SVM: add warning for mistmatch between AVIC state and AVIC access page state Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 7/8] KVM: SVM: call avic_vcpu_load/avic_vcpu_put when enabling/disabling AVIC Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 8/8] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use Maxim Levitsky
2021-07-18 12:13 ` Maxim Levitsky
2021-07-19 7:47 ` Vitaly Kuznetsov
2021-07-19 9:00 ` Maxim Levitsky
2021-07-19 9:23 ` Vitaly Kuznetsov
2021-07-19 9:58 ` Maxim Levitsky
2021-07-19 18:49 ` Sean Christopherson
2021-07-20 9:40 ` Maxim Levitsky
2021-07-22 9:12 ` KVM's support for non default APIC base Maxim Levitsky
2021-08-02 9:20 ` Maxim Levitsky
2021-08-06 21:55 ` Sean Christopherson
2021-08-09 9:40 ` Maxim Levitsky
2021-08-09 15:57 ` Sean Christopherson
2021-08-09 16:47 ` Jim Mattson
2021-08-10 20:42 ` Maxim Levitsky
2021-07-22 17:35 ` [PATCH v2 8/8] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use Maxim Levitsky
2021-07-22 19:06 ` Sean Christopherson
2021-07-27 13:05 ` Maxim Levitsky
2021-07-27 17:48 ` Ben Gardon
2021-07-27 18:17 ` Sean Christopherson
2021-07-29 14:10 ` Maxim Levitsky [this message]
2021-07-26 17:24 ` [PATCH v2 0/8] My AVIC patch queue Paolo Bonzini
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