similar to: [PATCH] virtio: Try to untangle DMA coherency

On Wed, Feb 01, 2017 at 08:09:21PM +0200, Michael S. Tsirkin wrote: > On Wed, Feb 01, 2017 at 12:25:57PM +0000, Robin Murphy wrote: > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > index 7e38ed79c3fc..961af25b385c 100644 > > --- a/drivers/virtio/virtio_ring.c > > +++ b/drivers/virtio/virtio_ring.c > > @@ -20,6 +20,7 @@ > >

On Wed, Feb 01, 2017 at 08:09:21PM +0200, Michael S. Tsirkin wrote: > On Wed, Feb 01, 2017 at 12:25:57PM +0000, Robin Murphy wrote: > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > index 7e38ed79c3fc..961af25b385c 100644 > > --- a/drivers/virtio/virtio_ring.c > > +++ b/drivers/virtio/virtio_ring.c > > @@ -20,6 +20,7 @@ > >

On 02/01/2017 08:19 PM, Michael S. Tsirkin wrote: > On Wed, Feb 01, 2017 at 06:27:09PM +0000, Will Deacon wrote: >> On Wed, Feb 01, 2017 at 08:09:21PM +0200, Michael S. Tsirkin wrote: >>> On Wed, Feb 01, 2017 at 12:25:57PM +0000, Robin Murphy wrote: >>>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c >>>> index

On 02/01/2017 08:19 PM, Michael S. Tsirkin wrote: > On Wed, Feb 01, 2017 at 06:27:09PM +0000, Will Deacon wrote: >> On Wed, Feb 01, 2017 at 08:09:21PM +0200, Michael S. Tsirkin wrote: >>> On Wed, Feb 01, 2017 at 12:25:57PM +0000, Robin Murphy wrote: >>>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c >>>> index

Whilst always using the DMA API is OK on ARM systems in most cases, there can be a problem if a hypervisor fails to tell its guest that a virtio device is cache-coherent. In that case, the guest will end up making non-cacheable mappings for DMA buffers (i.e. the vring), which, if the host is using a cacheable view of the same buffer on the other end, is not a recipe for success. It turns out that

Whilst always using the DMA API is OK on ARM systems in most cases, there can be a problem if a hypervisor fails to tell its guest that a virtio device is cache-coherent. In that case, the guest will end up making non-cacheable mappings for DMA buffers (i.e. the vring), which, if the host is using a cacheable view of the same buffer on the other end, is not a recipe for success. It turns out that

On Wed, Feb 01, 2017 at 12:25:57PM +0000, Robin Murphy wrote: > By forcing on DMA API usage for ARM systems, we have inadvertently > kicked open a hornets' nest in terms of cache-coherency. Namely that > unless the virtio device is explicitly described as capable of coherent > DMA by firmware, the DMA APIs on ARM and other DT-based platforms will > assume it is non-coherent.

On Wed, Feb 01, 2017 at 09:19:22PM +0200, Michael S. Tsirkin wrote: > On Wed, Feb 01, 2017 at 06:27:09PM +0000, Will Deacon wrote: > > On Wed, Feb 01, 2017 at 08:09:21PM +0200, Michael S. Tsirkin wrote: > > > I'd like to do that instead. It's fastboot doing the unreasonable thing > > > here and deviating from what every other legacy device without exception >

On Wed, Feb 01, 2017 at 09:19:22PM +0200, Michael S. Tsirkin wrote: > On Wed, Feb 01, 2017 at 06:27:09PM +0000, Will Deacon wrote: > > On Wed, Feb 01, 2017 at 08:09:21PM +0200, Michael S. Tsirkin wrote: > > > I'd like to do that instead. It's fastboot doing the unreasonable thing > > > here and deviating from what every other legacy device without exception >

On Wed, Feb 01, 2017 at 06:27:09PM +0000, Will Deacon wrote: > On Wed, Feb 01, 2017 at 08:09:21PM +0200, Michael S. Tsirkin wrote: > > On Wed, Feb 01, 2017 at 12:25:57PM +0000, Robin Murphy wrote: > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > > index 7e38ed79c3fc..961af25b385c 100644 > > > --- a/drivers/virtio/virtio_ring.c

On Wed, Feb 01, 2017 at 12:25:57PM +0000, Robin Murphy wrote: > By forcing on DMA API usage for ARM systems, we have inadvertently > kicked open a hornets' nest in terms of cache-coherency. Namely that > unless the virtio device is explicitly described as capable of coherent > DMA by firmware, the DMA APIs on ARM and other DT-based platforms will > assume it is non-coherent.

On Thu, Feb 02, 2017 at 01:34:03PM +0000, Robin Murphy wrote: > On 02/02/17 11:26, Will Deacon wrote: > > On Wed, Feb 01, 2017 at 09:19:22PM +0200, Michael S. Tsirkin wrote: > >> On Wed, Feb 01, 2017 at 06:27:09PM +0000, Will Deacon wrote: > >>> On Wed, Feb 01, 2017 at 08:09:21PM +0200, Michael S. Tsirkin wrote: > >>>> I'd like to do that instead.

On Thu, Feb 02, 2017 at 01:34:03PM +0000, Robin Murphy wrote: > On 02/02/17 11:26, Will Deacon wrote: > > On Wed, Feb 01, 2017 at 09:19:22PM +0200, Michael S. Tsirkin wrote: > >> On Wed, Feb 01, 2017 at 06:27:09PM +0000, Will Deacon wrote: > >>> On Wed, Feb 01, 2017 at 08:09:21PM +0200, Michael S. Tsirkin wrote: > >>>> I'd like to do that instead.

On Thu, Feb 02, 2017 at 01:34:03PM +0000, Robin Murphy wrote: > On 02/02/17 11:26, Will Deacon wrote: > > On Wed, Feb 01, 2017 at 09:19:22PM +0200, Michael S. Tsirkin wrote: > >> On Wed, Feb 01, 2017 at 06:27:09PM +0000, Will Deacon wrote: > >>> On Wed, Feb 01, 2017 at 08:09:21PM +0200, Michael S. Tsirkin wrote: > >>>> I'd like to do that instead.

On Thu, Feb 02, 2017 at 01:34:03PM +0000, Robin Murphy wrote: > On 02/02/17 11:26, Will Deacon wrote: > > On Wed, Feb 01, 2017 at 09:19:22PM +0200, Michael S. Tsirkin wrote: > >> On Wed, Feb 01, 2017 at 06:27:09PM +0000, Will Deacon wrote: > >>> On Wed, Feb 01, 2017 at 08:09:21PM +0200, Michael S. Tsirkin wrote: > >>>> I'd like to do that instead.

On Wed, Feb 08, 2017 at 01:58:10PM +0100, Alexander Graf wrote: > On 02/01/2017 08:19 PM, Michael S. Tsirkin wrote: > > On Wed, Feb 01, 2017 at 06:27:09PM +0000, Will Deacon wrote: > > > On Wed, Feb 01, 2017 at 08:09:21PM +0200, Michael S. Tsirkin wrote: > > > > On Wed, Feb 01, 2017 at 12:25:57PM +0000, Robin Murphy wrote: > > > > > diff --git

On Thu, Feb 02, 2017 at 04:40:49PM +0000, Will Deacon wrote: > On Thu, Feb 02, 2017 at 06:30:28PM +0200, Michael S. Tsirkin wrote: > > I am inclined to say, for 4.10 let's revert > > c7070619f3408d9a0dffbed9149e6f00479cf43b since what it fixes is not a > > regression in 4.10. > > No complaints there, as long as we can keep working to fix this for 4.11 > and

On Thu, Feb 02, 2017 at 04:40:49PM +0000, Will Deacon wrote: > On Thu, Feb 02, 2017 at 06:30:28PM +0200, Michael S. Tsirkin wrote: > > I am inclined to say, for 4.10 let's revert > > c7070619f3408d9a0dffbed9149e6f00479cf43b since what it fixes is not a > > regression in 4.10. > > No complaints there, as long as we can keep working to fix this for 4.11 > and

This reverts commit c7070619f3408d9a0dffbed9149e6f00479cf43b. This has been shown to regress on some ARM systems: by forcing on DMA API usage for ARM systems, we have inadvertently kicked open a hornets' nest in terms of cache-coherency. Namely that unless the virtio device is explicitly described as capable of coherent DMA by firmware, the DMA APIs on ARM and other DT-based platforms will

This reverts commit c7070619f3408d9a0dffbed9149e6f00479cf43b. This has been shown to regress on some ARM systems: by forcing on DMA API usage for ARM systems, we have inadvertently kicked open a hornets' nest in terms of cache-coherency. Namely that unless the virtio device is explicitly described as capable of coherent DMA by firmware, the DMA APIs on ARM and other DT-based platforms will