linux-kernel - Re: [PATCH 12/13] KVM: x86/mmu: Fast invalidation for TDP MMU

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Message-ID: <CANgfPd_wFMFQgqSG9gi5zo3=WMGVST-66DkNJTopvTBZoangmQ@mail.gmail.com>
Date:   Thu, 1 Apr 2021 09:50:05 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.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 12/13] KVM: x86/mmu: Fast invalidation for TDP MMU

On Thu, Apr 1, 2021 at 3:36 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 31/03/21 23:08, Ben Gardon wrote:
> >
> > +     if (is_tdp_mmu_enabled(kvm))
> > +             kvm_tdp_mmu_invalidate_roots(kvm);
> > +
> >       /*
> >        * Toggle mmu_valid_gen between '0' and '1'.  Because slots_lock is
> >        * held for the entire duration of zapping obsolete pages, it's
> > @@ -5451,9 +5454,6 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm)
> >
> >       kvm_zap_obsolete_pages(kvm);
> >
> > -     if (is_tdp_mmu_enabled(kvm))
> > -             kvm_tdp_mmu_zap_all(kvm);
> > -
>
> This is just cosmetic, but I'd prefer to keep the call to
> kvm_tdp_mmu_invalidate_roots at the original place, so that it's clear
> in the next patch that it's two separate parts because of the different
> locking requirements.

I'm not sure exactly what you mean and I could certainly do a better
job explaining in the commit description, but it's actually quite
important that kvm_tdp_mmu_invalidate_roots at least precede
kvm_zap_obsolete_pages as kvm_zap_obsolete_pages drops the lock and
yields. If kvm_tdp_mmu_invalidate_roots doesn't go first then vCPUs
could wind up dropping their ref on an old root and then picking it up
again before the last root had a chance to drop its ref.

Explaining in the description that kvm_tdp_mmu_zap_all is being
dropped because it is no longer necessary (as opposed to being moved)
might help make that cleaner.

Alternatively I could just leave kvm_tdp_mmu_zap_all and replace it in
the next patch.

>
> Paolo
>