Linux Virtualization - Jul 2019 - [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

On Thu, Jul 25, 2019 at 01:53:49PM +0200, Auger Eric
wrote:
> 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?
Its not normal.  I assume you use virtio-nmio?  Due to the mess with
the dma_mask being a pointer a lot of subsystems forgot to set a dma
mask up, and oddly enough seem to mostly get away with it.

Hi Christoph, Michael,

On 7/25/19 3:04 PM, Christoph Hellwig wrote:
> On Thu, Jul 25, 2019 at 01:53:49PM +0200, Auger Eric wrote:
>> 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?
> 
> Its not normal.  I assume you use virtio-nmio?  Due to the mess with
> the dma_mask being a pointer a lot of subsystems forgot to set a dma
> mask up, and oddly enough seem to mostly get away with it.
> 
No the issue is encountered with virtio-blk-pci

I think the problem is virtio_max_dma_size() is called from
virtblk_probe (virtio_blk.c) on the virtio<n> device and not the actual
virtio_pci_device which has a dma_mask set. I don't think the
virtio<n>
device ever has a dma_mask set.

We do not hit the guest crash on the virtio-net-pci device as the
virtio-net driver does not call virtio_max_dma_size() on the virtio<n>
device.

Does fd1068e1860e ("virtio-blk: Consider virtio_max_dma_size() for
maximum segment size") call virtio_max_dma_size() on the right device?

Thanks

Eric