linux-kernel - Re: [PATCH 2/2] TESTING! KVM: x86: add invalidate

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Message-ID: <20171203172447.GQ8063@redhat.com>
Date:   Sun, 3 Dec 2017 18:24:47 +0100
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Fabian Grünbichler 
        <f.gruenbichler@...xmox.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, kvm@...r.kernel.org,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [PATCH 2/2] TESTING! KVM: x86: add invalidate_range mmu notifier

On Fri, Dec 01, 2017 at 04:15:37PM +0100, Paolo Bonzini wrote:
> On 30/11/2017 19:05, Radim Krčmář wrote:
> > Does roughly what kvm_mmu_notifier_invalidate_page did before.
> > 
> > I am not certain why this would be needed.  It might mean that we have
> > another bug with start/end or just that I missed something.
> 
> I don't think this is needed, because we don't have shared page tables.
> My understanding is that without shared page tables, you can assume that
> all page modifications go through invalidate_range_start/end.  With
> shared page tables, there are additional TLB flushes to take care of,
> which require invalidate_range.

Agreed, invalidate_range only is ever needed if you the secondary MMU
(i.e. KVM) shares the same pagetables of the primary MMU in the
host. Only in such case we need a special secondary MMU invalidate in
the tlb gather before the page is freed because there's no way to
block the secondary MMU from walking the host pagetables in
invalidate_range_start.

In KVM case the secondary MMU always go through the shadow pagetables,
so all shadow pagetable invalidates can happen in
invalidate_range_start and patch 2/2 is not needed here.

Note that the host kernel could have always decided to call
invalidate_range_start/end and never to call invalidate_page even
before invalidate_page was removed.

So the problem in practice could only be noticed after the removal of
invalidate_page of course, but in more theoretical terms 1/2 is
actually fixing a longstanding bug. The removal of invalidate_page
made the lack of kvm_arch_mmu_notifier_invalidate_page call in
invalidate_range_start more apparent.

Thanks,
Andrea