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Message-ID:
<IA1PR12MB653270F7FD75C1B4DF8B90E8F27C2@IA1PR12MB6532.namprd12.prod.outlook.com>
Date: Wed, 31 Jan 2024 10:20:12 +0000
From: "Zhang, Julia" <Julia.Zhang@....com>
To: "Koenig, Christian" <Christian.Koenig@....com>, "Zhang, Julia"
<Julia.Zhang@....com>, Gurchetan Singh <gurchetansingh@...omium.org>, Chia-I
Wu <olvaffe@...il.com>, David Airlie <airlied@...hat.com>, Gerd Hoffmann
<kraxel@...hat.com>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>, "dri-devel@...ts.freedesktop.org"
<dri-devel@...ts.freedesktop.org>, "amd-gfx@...ts.freedesktop.org"
<amd-gfx@...ts.freedesktop.org>, "virtualization@...ts.linux-foundation.org"
<virtualization@...ts.linux-foundation.org>, "Deucher, Alexander"
<Alexander.Deucher@....com>, David Airlie <airlied@...il.com>, Erik Faye-Lund
<kusmabite@...il.com>, "Olsak, Marek" <Marek.Olsak@....com>, "Pelloux-Prayer,
Pierre-Eric" <Pierre-eric.Pelloux-prayer@....com>, "Huang, Honglei1"
<Honglei1.Huang@....com>, "Chen, Jiqian" <Jiqian.Chen@....com>, "Huang, Ray"
<Ray.Huang@....com>, David Stevens <stevensd@...omium.org>
Subject: Re: [PATCH v2 1/1] drm/virtio: Implement device_attach
On 2024/1/30 22:23, Christian König wrote:
> Am 30.01.24 um 12:16 schrieb Daniel Vetter:
>> On Tue, Jan 30, 2024 at 12:10:31PM +0100, Daniel Vetter wrote:
>>> On Mon, Jan 29, 2024 at 06:31:19PM +0800, Julia Zhang wrote:
>>>> As vram objects don't have backing pages and thus can't implement
>>>> drm_gem_object_funcs.get_sg_table callback. This removes drm dma-buf
>>>> callbacks in virtgpu_gem_map_dma_buf()/virtgpu_gem_unmap_dma_buf()
>>>> and implement virtgpu specific map/unmap/attach callbacks to support
>>>> both of shmem objects and vram objects.
>>>>
>>>> Signed-off-by: Julia Zhang <julia.zhang@....com>
>>>> ---
>>>> drivers/gpu/drm/virtio/virtgpu_prime.c | 40 +++++++++++++++++++++++---
>>>> 1 file changed, 36 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/virtio/virtgpu_prime.c b/drivers/gpu/drm/virtio/virtgpu_prime.c
>>>> index 44425f20d91a..b490a5343b06 100644
>>>> --- a/drivers/gpu/drm/virtio/virtgpu_prime.c
>>>> +++ b/drivers/gpu/drm/virtio/virtgpu_prime.c
>>>> @@ -49,11 +49,26 @@ virtgpu_gem_map_dma_buf(struct dma_buf_attachment *attach,
>>>> {
>>>> struct drm_gem_object *obj = attach->dmabuf->priv;
>>>> struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(obj);
>>>> + struct sg_table *sgt;
>>>> + int ret;
>>>> if (virtio_gpu_is_vram(bo))
>>>> return virtio_gpu_vram_map_dma_buf(bo, attach->dev, dir);
>>>> - return drm_gem_map_dma_buf(attach, dir);
>>>> + sgt = drm_prime_pages_to_sg(obj->dev,
>>>> + to_drm_gem_shmem_obj(obj)->pages,
>>>> + obj->size >> PAGE_SHIFT);
>>>> + if (IS_ERR(sgt))
>>>> + return sgt;
>>>> +
>>>> + ret = dma_map_sgtable(attach->dev, sgt, dir, DMA_ATTR_SKIP_CPU_SYNC);
>>>> + if (ret) {
>>>> + sg_free_table(sgt);
>>>> + kfree(sgt);
>>>> + return ERR_PTR(ret);
>>>> + }
>>>> +
>>>> + return sgt;
>>>> }
>>>> static void virtgpu_gem_unmap_dma_buf(struct dma_buf_attachment *attach,
>>>> @@ -63,12 +78,29 @@ static void virtgpu_gem_unmap_dma_buf(struct dma_buf_attachment *attach,
>>>> struct drm_gem_object *obj = attach->dmabuf->priv;
>>>> struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(obj);
>>>> + if (!sgt)
>>>> + return;
>>>> +
>>>> if (virtio_gpu_is_vram(bo)) {
>>>> virtio_gpu_vram_unmap_dma_buf(attach->dev, sgt, dir);
>>>> - return;
>>>> + } else {
>>>> + dma_unmap_sgtable(attach->dev, sgt, dir, DMA_ATTR_SKIP_CPU_SYNC);
>>>> + sg_free_table(sgt);
>>>> + kfree(sgt);
>>>> }
>>>> +}
>>>> +
>>>> +static int virtgpu_gem_device_attach(struct dma_buf *dma_buf,
>>>> + struct dma_buf_attachment *attach)
>>>> +{
>>>> + struct drm_gem_object *obj = attach->dmabuf->priv;
>>>> + struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(obj);
>>>> + int ret = 0;
>>>> +
>>>> + if (!virtio_gpu_is_vram(bo) && obj->funcs->pin)
>>>> + ret = obj->funcs->pin(obj);
>>>> - drm_gem_unmap_dma_buf(attach, sgt, dir);
>>>> + return ret;
>>> This doesn't look like what I've expected. There should be no need to
>>> change the map/unmap functions, especially not for the usual gem bo case.
>>> We should definitely keep using the exact same code for that. Instead all
>>> I expected is roughly
>>>
>>> virtgpu_gem_device_attach()
>>> {
>>> if (virtio_gpu_is_vram(bo)) {
>>> if (can_access_virtio_vram_directly(attach->dev)
>>> return 0;
>>> else
>>> return -EBUSY;
>>> } else {
>>> return drm_gem_map_attach();
>>> }
>>> }
>>>
>>> Note that I think can_access_virtio_vram_directly() needs to be
>>> implemented first. I'm not even sure it's possible, might be that all the
>>> importers need to set the attachment->peer2peer flag. Which is why this
>>> thing exists really. But that's a pile more work to do.
>
Hi Sima, Christian,
> Yeah, that is really just speculative. All importers need to set the peer2peer flag just in case.
I see, I will modify this.
>
> What happens under the hood is that IOMMU redirects the "VRAM" memory access to whatever address the DMA-buf on the host is pointing to (system, VRAM, doorbell, IOMMU, whatever).
>
> I'm also not 100% sure if all the cache snooping is done correctly in all cases, but for now it seems to work.
>>>>
>>> Frankly the more I look at the original patch that added vram export
>>> support the more this just looks like a "pls revert, this is just too
>>> broken".
>> The commit I mean is this one: ea5ea3d8a117 ("drm/virtio: support mapping
>> exported vram"). The commit message definitely needs to cite that one, and
>> also needs a cc: stable because not rejecting invalid imports is a pretty
>> big deal.
>
> Yeah, I've pointed out that commit in an internal discussion as well. I was just not aware that it's that severely broken.
>
Yeah we have mentioned this patch before, but I don't totally understand why this is too broken. Without exporting vram objects, dGPU prime feature would not be realized.
Would you mind to explain more about it. Thanks!
Best regards,
Julia
> Regards,
> Christian.
>
>>
>> Also adding David.
>> -Sima
>>
>>> We should definitely not open-code any functions for the gem_bo export
>>> case, which your patch seems to do? Or maybe I'm just extremely confused.
>>> -Sima
>>>
>>>> static const struct virtio_dma_buf_ops virtgpu_dmabuf_ops = {
>>>> @@ -83,7 +115,7 @@ static const struct virtio_dma_buf_ops virtgpu_dmabuf_ops = {
>>>> .vmap = drm_gem_dmabuf_vmap,
>>>> .vunmap = drm_gem_dmabuf_vunmap,
>>>> },
>>>> - .device_attach = drm_gem_map_attach,
>>>> + .device_attach = virtgpu_gem_device_attach,
>>>> .get_uuid = virtgpu_virtio_get_uuid,
>>>> };
>>>> --
>>>> 2.34.1
>>>>
>>> --
>>> Daniel Vetter
>>> Software Engineer, Intel Corporation
>>> http://blog.ffwll.ch
>
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