linux-kernel - Re: [PATCH v2 1/6] KVM: x86/mmu: Tag disallowed NX huge pages even if they're not tracked

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Message-ID: <Yt8uwMt/3JPrSWM9@google.com>
Date:   Tue, 26 Jul 2022 00:01:04 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v2 1/6] KVM: x86/mmu: Tag disallowed NX huge pages even
 if they're not tracked

On Mon, Jul 25, 2022, David Matlack wrote:
> On Mon, Jul 25, 2022 at 4:26 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > The only scenario that jumps to mind is the non-coherent DMA with funky MTRRs
> > case.  There might be others, but it's been a while since I wrote this...
> >
> > The MTRRs are per-vCPU (KVM really should just track them as per-VM, but whatever),
> > so it's possible that KVM could encounter a fault with a lower fault->req_level
> > than a previous fault that set nx_huge_page_disallowed=true (and added the page
> > to the possible_nx_huge_pages list because it had a higher req_level).
> 
> But in that case the lower level SP would already have been installed,
> so we wouldn't end up calling account_nx_huge_page() and getting to
> this point. (account_nx_huge_page() is only called when linking in an
> SP.)

Hrm, true.  I'm 99% certain past me was just maintaining the existing logic in
account_huge_nx_page()

	if (sp->lpage_disallowed)
		return;

Best thing might be to turn that into a WARN as the first patch?

> Maybe account_nx_huge_page() needs to be pulled out and called for
> every SP on the walk during a fault?

Eh, not worth it, the MTRR thing is bogus anyways, e.g. if vCPUs have different
MTRR settings and one vCPU allows a huge page but the other does not, KVM will
may or may not install a huge page depending on which vCPU faults in the page.