linux-kernel - Re: [PATCH 2/2] virtio/virtio_ring: Fix the dma_max_mapping

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Date:   Thu, 25 Jul 2019 13:53:49 +0200
From:   Auger Eric <eric.auger@...hat.com>
To:     Christoph Hellwig <hch@....de>, Robin Murphy <robin.murphy@....com>
Cc:     eric.auger.pro@...il.com, m.szyprowski@...sung.com, mst@...hat.com,
        jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] virtio/virtio_ring: Fix the dma_max_mapping_size call

Hi,

On 7/23/19 5:38 PM, Christoph Hellwig wrote:
> On Mon, Jul 22, 2019 at 04:36:09PM +0100, Robin Murphy wrote:
>>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>>> index c8be1c4f5b55..37c143971211 100644
>>> --- a/drivers/virtio/virtio_ring.c
>>> +++ b/drivers/virtio/virtio_ring.c
>>> @@ -262,7 +262,7 @@ size_t virtio_max_dma_size(struct virtio_device *vdev)
>>>   {
>>>   	size_t max_segment_size = SIZE_MAX;
>>>   -	if (vring_use_dma_api(vdev))
>>> +	if (vring_use_dma_api(vdev) && vdev->dev.dma_mask)
>>
>> Hmm, might it make sense to roll that check up into vring_use_dma_api() 
>> itself? After all, if the device has no mask then it's likely that other 
>> DMA API ops wouldn't really work as expected either.
> 
> Makes sense to me.
> 

I am confused: if vring_use_dma_api() returns false if the dma_mask is
unset (ie. vring_use_dma_api() returns false), the virtio-blk-pci device
will not be able to get translated addresses and won't work properly.

The patch above allows the dma api to be used and only influences the
max_segment_size and it works properly.

So is it normal the dma_mask is unset in my case?

Thanks

Eric