Re: [RFC] virtio: support VIRTIO_F_IO_BARRIER

[Tiwei Bie]

On Thu, May 03, 2018 at 10:06:52AM +0100, Stefan Hajnoczi wrote:
> On Thu, May 03, 2018 at 10:59:55AM +0800, Tiwei Bie wrote:
> > This patch introduces the support for VIRTIO_F_IO_BARRIER.
> > When this feature is negotiated, driver will use the barriers
> > suitable for hardware devices.
> > 
> > Signed-off-by: Tiwei Bie <tiwei.bie@xxxxxxxxx>
> 
> I should have thought of this earlier, but why is a new feature bit
> necessary?  If a hardware virtio device is in use, then the device
> should already negotiate VIRTIO_F_IOMMU_PLATFORM (i.e. use DMA APIs and
> IOMMU callbacks).
> 
> Does disabling weak_barriers when VIRTIO_F_IOMMU_PLATFORM is set solve
> the problem?

The VIRTIO_F_IOMMU_PLATFORM feature can be set when the
device is implemented in software. And I think we don't
want the performance drop in this case.

Best regards,
Tiwei Bie