Re: [PATCH] xen: introduce xen_vring_use_dma

[Stefano Stabellini]

On Sat, 11 Jul 2020, Michael S. Tsirkin wrote:
> On Fri, Jul 10, 2020 at 10:23:22AM -0700, Stefano Stabellini wrote:
> > Sorry for the late reply -- a couple of conferences kept me busy.
> > 
> > 
> > On Wed, 1 Jul 2020, Michael S. Tsirkin wrote:
> > > On Wed, Jul 01, 2020 at 10:34:53AM -0700, Stefano Stabellini wrote:
> > > > Would you be in favor of a more flexible check along the lines of the
> > > > one proposed in the patch that started this thread:
> > > > 
> > > >     if (xen_vring_use_dma())
> > > >             return true;
> > > > 
> > > > 
> > > > xen_vring_use_dma would be implemented so that it returns true when
> > > > xen_swiotlb is required and false otherwise.
> > > 
> > > Just to stress - with a patch like this virtio can *still* use DMA API
> > > if PLATFORM_ACCESS is set. So if DMA API is broken on some platforms
> > > as you seem to be saying, you guys should fix it before doing something
> > > like this..
> > 
> > Yes, DMA API is broken with some interfaces (specifically: rpmesg and
> > trusty), but for them PLATFORM_ACCESS is never set. That is why the
> > errors weren't reported before. Xen special case aside, there is no
> > problem under normal circumstances.
> 
> So why not fix DMA API? Then this patch is not needed.

It looks like the conversation is going in circles :-)

I tried to explain the reason why, even if we fixed the DMA API to work
with rpmesg and trusty, we still need this patch with the following
email:  https://marc.info/?l=linux-kernel&m=159347446709625&w=2


At that time it looked like you agreed that we needed to improve this
check?  (https://marc.info/?l=linux-kernel&m=159363662418250&w=2)