linux-kernel - Re: [PATCH] KVM: Move gfn_to_pfn_cache invalidation to invalidate_range

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Message-ID: <592b598ecd3a8cf7188750d20582899d03489230.camel@infradead.org>
Date: Thu, 22 Aug 2024 10:00:49 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Mushahid Hussain
 <hmushi@...zon.co.uk>,  Vitaly Kuznetsov <vkuznets@...hat.com>, Wanpeng Li
 <wanpengli@...cent.com>, Jim Mattson <jmattson@...gle.com>,  Joerg Roedel
 <joro@...tes.org>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Mingwei Zhang <mizhang@...gle.com>, Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH] KVM: Move gfn_to_pfn_cache invalidation to
 invalidate_range_end hook

On Tue, 2024-08-06 at 07:03 -0700, Sean Christopherson wrote:
> On Tue, Aug 06, 2024, David Woodhouse wrote:
> > On Mon, 2024-08-05 at 17:45 -0700, Sean Christopherson wrote:
> > > On Mon, Aug 05, 2024, David Woodhouse wrote:
> > > > From: David Woodhouse <dwmw@...zon.co.uk>
> > > Servicing guest pages faults has the same problem, which is why
> > > mmu_invalidate_retry_gfn() was added.  Supporting hva-only GPCs made our lives a
> > > little harder, but not horrifically so (there are ordering differences regardless).
> > > 
> > > Woefully incomplete, but I think this is the gist of what you want:
> > 
> > Hm, maybe. It does mean that migration occurring all through memory
> > (indeed, just one at top and bottom of guest memory space) would
> > perturb GPCs which remain present.
> 
> If that happens with a real world VMM, and it's not a blatant VMM goof, then we
> can fix KVM.  The stage-2 page fault path hammers the mmu_notifier retry logic
> far more than GPCs, so if a range-based check is inadequate for some use case,
> then we definitely need to fix both.

FWIW this turned out to be because someone used C++ where they
shouldn't have done. Opinion is divided on whether to call that a
'blatant VMM goof'.

But if you use std::vector for something that's modified that many
times a second, and free() implicitly calls madvise(MADV_DONTNEED)
every time, you get what you deserve :)

We should still fix it in the kernel.


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