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SubjectRe: Re: [PATCH] virtio_ring: use u32 for virtio_max_dma_size
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On 5/10/23 11:26, Xuan Zhuo wrote:
> On Wed, 10 May 2023 10:54:37 +0800, zhenwei pi <pizhenwei@bytedance.com> wrote:
>> Both split ring and packed ring use 32bits to describe the length of
>> a descriptor: see struct vring_desc and struct vring_packed_desc.
>> This means the max segment size supported by virtio is U32_MAX.
>>
>> An example of virtio_max_dma_size in virtio_blk.c:
>> u32 v, max_size;
>>
>> max_size = virtio_max_dma_size(vdev); -> implicit convert
>> err = virtio_cread_feature(vdev, VIRTIO_BLK_F_SIZE_MAX,
>> struct virtio_blk_config, size_max, &v);
>> max_size = min(max_size, v);
>>
>> There is a risk during implicit convert here, once virtio_max_dma_size
>> returns 4G, max_size becomes 0.
>>
>> Fixes: e6d6dd6c875e ("virtio: Introduce virtio_max_dma_size()")
>> Cc: Joerg Roedel <jroedel@suse.de>
>> Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
>> ---
>> drivers/virtio/virtio_ring.c | 12 ++++++++----
>> include/linux/virtio.h | 2 +-
>> 2 files changed, 9 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>> index c5310eaf8b46..55cfecf030a1 100644
>> --- a/drivers/virtio/virtio_ring.c
>> +++ b/drivers/virtio/virtio_ring.c
>> @@ -289,12 +289,16 @@ static bool vring_use_dma_api(const struct virtio_device *vdev)
>> return false;
>> }
>>
>> -size_t virtio_max_dma_size(const struct virtio_device *vdev)
>> +u32 virtio_max_dma_size(const struct virtio_device *vdev)
>
>
> LGTM
>
> But, should we change the parameter to vq, then use the dma_dev?
>
> @Jason
>
> Thanks.
>

The max DMA size is a attribute of a virtio device rather than any VQ,
so I guess virtio_max_dma_size(const struct virtio_device *vdev) is clear.

On the other hand, if changing the parameter to vq, we need select a VQ,
then the question is:
1, which VQ to select? VQ0 or a random one? this leads confusing.
2, The virtio spec defines: Each device can have zero or more virtqueues


>
>> {
>> - size_t max_segment_size = SIZE_MAX;
>> + u32 max_segment_size = U32_MAX;
>>
>> - if (vring_use_dma_api(vdev))
>> - max_segment_size = dma_max_mapping_size(vdev->dev.parent);
>> + if (vring_use_dma_api(vdev)) {
>> + size_t max_dma_size = dma_max_mapping_size(vdev->dev.parent);
>> +
>> + if (max_dma_size < max_segment_size)
>> + max_segment_size = max_dma_size;
>> + }
>>
>> return max_segment_size;
>> }
>> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
>> index b93238db94e3..1a605f408329 100644
>> --- a/include/linux/virtio.h
>> +++ b/include/linux/virtio.h
>> @@ -147,7 +147,7 @@ int virtio_device_restore(struct virtio_device *dev);
>> #endif
>> void virtio_reset_device(struct virtio_device *dev);
>>
>> -size_t virtio_max_dma_size(const struct virtio_device *vdev);
>> +u32 virtio_max_dma_size(const struct virtio_device *vdev);
>>
>> #define virtio_device_for_each_vq(vdev, vq) \
>> list_for_each_entry(vq, &vdev->vqs, list)
>> --
>> 2.20.1
>>

--
zhenwei pi

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