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

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Date:   Thu, 1 Apr 2021 09:50:09 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <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 12/13] KVM: x86/mmu: Fast invalidation for TDP MMU

On Wed, Mar 31, 2021 at 3:27 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, Mar 31, 2021, Ben Gardon wrote:
> > Provide a real mechanism for fast invalidation by marking roots as
> > invalid so that their reference count will quickly fall to zero
> > and they will be torn down.
> >
> > One negative side affect of this approach is that a vCPU thread will
> > likely drop the last reference to a root and be saddled with the work of
> > tearing down an entire paging structure. This issue will be resolved in
> > a later commit.
> >
> > Signed-off-by: Ben Gardon <bgardon@...gle.com>
> > ---
>
> ...
>
> > +/*
> > + * This function depends on running in the same MMU lock cirical section as
> > + * kvm_reload_remote_mmus. Since this is in the same critical section, no new
> > + * roots will be created between this function and the MMU reload signals
> > + * being sent.
>
> Eww.  That goes beyond just adding a lockdep assertion here.  I know you want to
> isolate the TDP MMU as much as possible, but this really feels like it should be
> open coded in kvm_mmu_zap_all_fast().  And assuming this lands after as_id is
> added to for_each_tdp_mmu_root(), it's probably easier to open code anyways, e.g.
> use list_for_each_entry() directly instead of bouncing through an iterator.

Yeah, that's fair. I'll open-code it here. I agree that it will remove
confusion from the function, though it would be nice to be able to use
for_each_tdp_mmu_root for the lockdep and rcu annotations.


>
> > + */
> > +void kvm_tdp_mmu_invalidate_roots(struct kvm *kvm)
> > +{
> > +     struct kvm_mmu_page *root;
> > +
> > +     for_each_tdp_mmu_root(kvm, root)
> > +             root->role.invalid = true;
> > +}