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

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Date:   Fri, 18 Jan 2019 09:40:00 +0000
From:   Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     Gerd Hoffmann <kraxel@...hat.com>,
        Oleksandr Andrushchenko <andr2000@...il.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "daniel.vetter@...el.com" <daniel.vetter@...el.com>,
        "jgross@...e.com" <jgross@...e.com>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "noralf@...nnes.org" <noralf@...nnes.org>
Subject: Re: [PATCH v2] drm/xen-front: Make shmem backed display buffer
 coherent

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

Thank you,
Oleksandr