Message-ID: <CAD=HUj6gjqthaenpZUr2Lci3WJW86OWe559YrDWiUSVfZBNHiQ@mail.gmail.com>
Date: Tue, 6 Aug 2024 08:44:08 +0900
From: David Stevens <stevensd@...omium.org>
To: Alex Bennée <alex.bennee@...aro.org>
Cc: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Yu Zhang <yu.c.zhang@...ux.intel.com>, Isaku Yamahata <isaku.yamahata@...il.com>, 
	Zhi Wang <zhi.wang.linux@...il.com>, Maxim Levitsky <mlevitsk@...hat.com>, kvmarm@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v11 0/8] KVM: allow mapping non-refcounted pages

On Wed, Jul 31, 2024 at 8:41 PM Alex Bennée <alex.bennee@...aro.org> wrote:
>
> Sean Christopherson <seanjc@...gle.com> writes:
>
> > On Thu, Feb 29, 2024, David Stevens wrote:
> >> From: David Stevens <stevensd@...omium.org>
> >>
> >> This patch series adds support for mapping VM_IO and VM_PFNMAP memory
> >> that is backed by struct pages that aren't currently being refcounted
> >> (e.g. tail pages of non-compound higher order allocations) into the
> >> guest.
> >>
> >> Our use case is virtio-gpu blob resources [1], which directly map host
> >> graphics buffers into the guest as "vram" for the virtio-gpu device.
> >> This feature currently does not work on systems using the amdgpu driver,
> >> as that driver allocates non-compound higher order pages via
> >> ttm_pool_alloc_page().
> >>
> >> First, this series replaces the gfn_to_pfn_memslot() API with a more
> >> extensible kvm_follow_pfn() API. The updated API rearranges
> >> gfn_to_pfn_memslot()'s args into a struct and where possible packs the
> >> bool arguments into a FOLL_ flags argument. The refactoring changes do
> >> not change any behavior.
> >>
> >> From there, this series extends the kvm_follow_pfn() API so that
> >> non-refconuted pages can be safely handled. This invloves adding an
> >> input parameter to indicate whether the caller can safely use
> >> non-refcounted pfns and an output parameter to tell the caller whether
> >> or not the returned page is refcounted. This change includes a breaking
> >> change, by disallowing non-refcounted pfn mappings by default, as such
> >> mappings are unsafe. To allow such systems to continue to function, an
> >> opt-in module parameter is added to allow the unsafe behavior.
> >>
> >> This series only adds support for non-refcounted pages to x86. Other
> >> MMUs can likely be updated without too much difficulty, but it is not
> >> needed at this point. Updating other parts of KVM (e.g. pfncache) is not
> >> straightforward [2].
> >
> > FYI, on the off chance that someone else is eyeballing this, I am working on
> > revamping this series.  It's still a ways out, but I'm optimistic that we'll be
> > able to address the concerns raised by Christoph and Christian, and maybe even
> > get KVM out of the weeds straightaway (PPC looks thorny :-/).
>
> I've applied this series to the latest 6.9.x while attempting to
> diagnose some of the virtio-gpu problems it may or may not address.
> However launching KVM guests keeps triggering a bunch of BUGs that
> eventually leave a hung guest:
>

Likely the same issue as [1]. Commit d02c357e5bfa added another call
to kvm_release_pfn_clean() in kvm_faultin_pfn(), which ends up
releasing a reference that is no longer being taken. If you replace
that with kvm_set_page_accessed() instead, then things should work
again. I didn't send out a rebased version of the series, since Sean's
work supersedes my series.

-David

[1] https://lore.kernel.org/lkml/15865985-4688-4b7e-9f2d-89803adb8f5b@collabora.com/

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