linux-kernel - Re: [PATCH] drm/virtio: fix host visible memory detection in virtio-gpu

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Message-ID: <20250827093320-mutt-send-email-mst@kernel.org>
Date: Wed, 27 Aug 2025 09:33:27 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc: Honglei Huang <honglei1.huang@....com>,
	David Airlie <airlied@...hat.com>,
	Gerd Hoffmann <kraxel@...hat.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Simona Vetter <simona@...ll.ch>,
	Gurchetan Singh <gurchetansingh@...omium.org>,
	Chia-I Wu <olvaffe@...il.com>, dri-devel@...ts.freedesktop.org,
	virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/virtio: fix host visible memory detection in
 virtio-gpu

On Wed, Aug 27, 2025 at 03:52:05PM +0300, Dmitry Osipenko wrote:
> On 8/27/25 11:12, Honglei Huang wrote:
> > From: Honglei Huang <Honglei1.Huang@....com>
> > 
> > Commit 206cc44588f7 ("virtio: reject shm region if length is zero")
> > enhanced the validation in virtio_get_shm_region() by adding a check
> > for a zero-length shared memory region.
> > 
> > It is performed before the underlying transport's .get_shm_region()
> > implementation is called. This creates an issue in the virtio-gpu
> > driver, where the `region` struct is part of a larger structure
> > that is zero-initialized by drmm_kzalloc().
> > 
> > Consequently, the `len` field is 0 at the time of the check, causing
> > virtio_get_shm_region() to return false prematurely. This prevents the
> > host visible memory feature from being enabled, even when the device
> > supports it.
> > 
> > To resolve this, this patch bypasses the inline helper and calls the
> > underlying vdev->config->get_shm_region() function pointer directly.
> > This ensures that the region's parameters are checked only after they
> > have been populated by the transport, aligning with the intended logic.
> > 
> > Signed-off-by: Honglei Huang <Honglei1.Huang@....com>
> > ---
> >  drivers/gpu/drm/virtio/virtgpu_kms.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c b/drivers/gpu/drm/virtio/virtgpu_kms.c
> > index 7dfb2006c561..ed5981248302 100644
> > --- a/drivers/gpu/drm/virtio/virtgpu_kms.c
> > +++ b/drivers/gpu/drm/virtio/virtgpu_kms.c
> > @@ -174,8 +174,10 @@ int virtio_gpu_init(struct virtio_device *vdev, struct drm_device *dev)
> >  	if (virtio_has_feature(vgdev->vdev, VIRTIO_GPU_F_RESOURCE_BLOB)) {
> >  		vgdev->has_resource_blob = true;
> >  	}
> > -	if (virtio_get_shm_region(vgdev->vdev, &vgdev->host_visible_region,
> > -				  VIRTIO_GPU_SHM_ID_HOST_VISIBLE)) {
> > +	if (vgdev->vdev->config->get_shm_region &&
> > +	    vgdev->vdev->config->get_shm_region(
> > +		    vgdev->vdev, &vgdev->host_visible_region,
> > +		    VIRTIO_GPU_SHM_ID_HOST_VISIBLE)) {
> >  		if (!devm_request_mem_region(&vgdev->vdev->dev,
> >  					     vgdev->host_visible_region.addr,
> >  					     vgdev->host_visible_region.len,
> 
> Hi, virtio_get_shm_region() change has been reverted by [1]. Don't think
> anything else needs to be done.
> 
> [1]
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?h=next-20250827&id=ced17ee32a9988b8a260628e7c31a100d7dc082e
> 
> +cc Michael Tsirkin
> 
> Might be only good to send a stable kernel PR with that revert. I see
> patch available only in linux-next, while stable kernels need to be
> fixed sooner.

sooner than what?