Re: [Xen-devel] [PATCH v2] drm/xen-front: Make shmem backed display buffer coherent

[Julien Grall <julien.grall@xxxxxxx>]

(+ Stefano)

Hi,

Sorry for jumping late in the conversation.

On 18/01/2019 09:40, Oleksandr Andrushchenko wrote:
> On 1/17/19 11:18 AM, Christoph Hellwig wrote:
> > On Wed, Jan 16, 2019 at 06:43:29AM +0000, Oleksandr Andrushchenko wrote:
> > > > This whole issue keeps getting more and more confusing.
> > > Well, I don't really do DMA here, but instead the buffers in
> > > question are shared with other Xen domain, so effectively it
> > > could be thought of some sort of DMA here, where the "device" is
> > > that remote domain. If the buffers are not flushed then the
> > > remote part sees some inconsistency which in my case results
> > > in artifacts on screen while displaying the buffers.
> > > When buffers are allocated via DMA API then there are no artifacts;
> > > if buffers are allocated with shmem + DMA mapping then there are no
> > > artifacts as well.
> > > The only offending use-case is when I use shmem backed buffers,
> > > but do not flush them
> > The right answer would be to implement cache maintainance hooks for
> > this case in the Xen arch code.  These would basically look the same
> > as the low-level cache maintainance used by the DMA ops, but without
> > going through the DMA mapping layer, in fact they should probably
> > reuse the same low-level assembly routines.
> >
> > I don't think this is the first usage of such Xen buffer sharing, so
> > what do the other users do?
> I'll have to get even deeper into it. Initially I
> looked at the code, but didn't find anything useful.
> Or maybe I have just overlooked obvious things there
From Xen on Arm ABI:

"All memory which is shared with other entities in the system
(including the hypervisor and other guests) must reside in memory
which is mapped as Normal Inner Write-Back Outer Write-Back Inner-Shareable.
This applies to:
  - hypercall arguments passed via a pointer to guest memory.
  - memory shared via the grant table mechanism (including PV I/O
    rings etc).
  - memory shared with the hypervisor (struct shared_info, struct
    vcpu_info, the grant table, etc).
"

So you should not need any cache maintenance here. Can you provide more details 
on the memory attribute you use for memory shared in both the backend and frontend?
Cheers,

> Thank you,
> Oleksandr
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--
Julien Grall

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