linux-kernel - Re: [PATCH v11 0/8] KVM: allow mapping non-refcounted pages

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Message-ID: <CAD=HUj7fT2CVXLfi5mty0rSzpG_jK9fhcKYGQnTf_H8Hg-541Q@mail.gmail.com>
Date: Wed, 13 Mar 2024 13:55:20 +0900
From: David Stevens <stevensd@...omium.org>
To: Christoph Hellwig <hch@...radead.org>
Cc: Christian Koenig <christian.koenig@....com>, 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 Thu, Feb 29, 2024 at 10:36 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Thu, Feb 29, 2024 at 11:57:51AM +0900, David Stevens wrote:
> > 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().
>
> .. and just as last time around that is still the problem that needs
> to be fixed instead of creating a monster like this to map
> non-refcounted pages.
>

Patches to amdgpu to have been NAKed [1] with the justification that
using non-refcounted pages is working as intended and KVM is in the
wrong for wanting to take references to pages mapped with VM_PFNMAP
[2].

The existence of the VM_PFNMAP implies that the existence of
non-refcounted pages is working as designed. We can argue about
whether or not VM_PFNMAP should exist, but until VM_PFNMAP is removed,
KVM should be able to handle it. Also note that this is not adding a
new source of non-refcounted pages, so it doesn't make removing
non-refcounted pages more difficult, if the kernel does decide to go
in that direction.

-David

[1] https://lore.kernel.org/lkml/8230a356-be38-f228-4a8e-95124e8e8db6@amd.com/
[2] https://lore.kernel.org/lkml/594f1013-b925-3c75-be61-2d649f5ca54e@amd.com/