linux-kernel - Re: `virtio: reject shm region if length is zero` breaks virtio-gpu host

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Message-ID: <20250806164708-mutt-send-email-mst@kernel.org>
Date: Wed, 6 Aug 2025 16:47:27 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Igor Torrente <igor.torrente@...labora.com>
Cc: samimdko <sami.md.ko@...il.com>,
	virtualization <virtualization@...ts.linux.dev>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	eperezma <eperezma@...hat.com>,
	xuanzhuo <xuanzhuo@...ux.alibaba.com>,
	jasowang <jasowang@...hat.com>,
	Dmitrii Osipenko <dmitry.osipenko@...labora.com>
Subject: Re: `virtio: reject shm region if length is zero` breaks virtio-gpu
 host_visible feature

On Wed, Aug 06, 2025 at 03:34:55PM -0300, Igor Torrente wrote:
> Hello,
> 
> The commit `206cc44588f7 virtio: reject shm region if length is zero` breaks the Virtio-gpu `host_visible` feature.
> 
> As you can see in the snippet below, host_visible_region is zero because of the `kzalloc`. 
> It's using the `vm_get_shm_region` (drivers/virtio/virtio_mmio.c:536) to read the `addr` and `len` from qemu/crosvm.
> 
> ```
> drivers/gpu/drm/virtio/virtgpu_kms.c
> 132         vgdev = drmm_kzalloc(dev, sizeof(struct virtio_gpu_device), GFP_KERNEL);
> [...]
> 177         if (virtio_get_shm_region(vgdev->vdev, &vgdev->host_visible_region, 
> 178                                   VIRTIO_GPU_SHM_ID_HOST_VISIBLE)) {
> ```
> Now it always fails.
> 
> I don't know exactly what issue this patch is trying to solve, but right now Virtio-gpu relies upon the previous behavior. 
> Can we just revert it?
> 
> BR,
> ----
> Igor Torrente


Pls go ahead and post the revert. We'll see what happens.

-- 
MST