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

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Date:   Thu, 25 Jul 2019 18:32:14 +0200
From:   Auger Eric <eric.auger@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Robin Murphy <robin.murphy@....com>, 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 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