linux-kernel - Re: KVM nonblocking MMU notifier with KVM_GUEST_USES

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Message-ID: <ZQiR8IpqOZrOpzHC@google.com>
Date:   Mon, 18 Sep 2023 11:07:44 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        kernel list <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: KVM nonblocking MMU notifier with KVM_GUEST_USES_PFN looks racy
 [but is currently unused]

On Mon, Sep 18, 2023, Jann Horn wrote:
> Hi!
> 
> I haven't tested this and might be missing something, but I think that
> the MMU notifier for KVM_GUEST_USES_PFN pfncache is currently a bit
> broken. Except that nothing seems to actually use KVM_GUEST_USES_PFN,
> so currently it's not actually a problem?

Yeah, the implementation is busted, and IMO the entire concept is a dead-end[1].
David Steven's series[2] doesn't actually rip out KVM_GUEST_USES_PFN, I'll add
that to the todo list.  There are no users, and I don't expect any to come along,
precisely because it's busted :-)

[1] https://lore.kernel.org/all/ZBEEQtmtNPaEqU1i@google.com
[2] https://lkml.kernel.org/r/20230911021637.1941096-1-stevensd%40google.com