From: Oleksandr Andrushchenko <andr2000@gmail.com>
To: Gerd Hoffmann <kraxel@redhat.com>,
"Oleksandr_Andrushchenko@epam.com"
<Oleksandr_Andrushchenko@epam.com>
Cc: "Noralf Trønnes" <noralf@tronnes.org>,
xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
dri-devel@lists.freedesktop.org, daniel.vetter@intel.com,
jgross@suse.com, boris.ostrovsky@oracle.com
Subject: Re: [PATCH] drm/xen-front: Make shmem backed display buffer coherent
Date: Thu, 20 Dec 2018 13:19:04 +0200 [thread overview]
Message-ID: <3e24ecc5-25e1-3d5e-2092-daa95ae36cba@gmail.com> (raw)
In-Reply-To: <20181219141000.k426c7o6ncsdzrn5@sirius.home.kraxel.org>
On 12/19/18 4:10 PM, Gerd Hoffmann wrote:
> Hi,
>
>>> Sure this actually helps? It's below 4G in guest physical address
>>> space, so it can be backed by pages which are actually above 4G in host
>>> physical address space ...
>> Yes, you are right here. This is why I wrote about the IOMMU
>> and other conditions. E.g. you can have a device which only
>> expects 32-bit, but thanks to IOMMU it can access pages above
>> 4GiB seamlessly. So, this is why I *hope* that this code *may* help
>> such devices. Do you think I don't need that and have to remove?
> I would try without that, and maybe add a runtime option (module
> parameter) later if it turns out some hardware actually needs that.
> Devices which can do 32bit DMA only become less and less common these
> days.
Good point, will remove then
>>>>>> + if (!dma_map_sg(dev->dev, xen_obj->sgt->sgl, xen_obj->sgt->nents,
>>>>>> + DMA_BIDIRECTIONAL)) {
>>>>> Are you using the DMA streaming API as a way to flush the caches?
>>>> Yes
>>>>> Does this mean that GFP_USER isn't making the buffer coherent?
>>>> No, it didn't help. I had a question [1] if there are any other better way
>>>> to achieve the same, but didn't have any response yet. So, I implemented
>>>> it via DMA API which helped.
>>> set_pages_array_*() ?
>>>
>>> See arch/x86/include/asm/set_memory.h
>> Well, x86... I am on arm which doesn't define that...
> Oh, arm. Maybe ask on a arm list then. I know on arm you have to care
> about caching a lot more, but that also is where my knowledge ends ...
>
> Using dma_map_sg for cache flushing looks like a sledge hammer approach
> to me.
It is. This is why I am so unsure this is way to go
> But maybe it is needed to make xen flush the caches (xen guests
> have their own dma mapping implementation, right? Or is this different
> on arm than on x86?).
I'll try to figure out
> cheers,
> Gerd
>
Thank you,
Oleksandr
next prev parent reply other threads:[~2018-12-20 11:19 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-27 10:32 [PATCH] drm/xen-front: Make shmem backed display buffer coherent Oleksandr Andrushchenko
2018-12-13 10:17 ` Oleksandr Andrushchenko
2018-12-13 15:48 ` Daniel Vetter
2018-12-14 7:09 ` Oleksandr Andrushchenko
2018-12-14 8:35 ` Daniel Vetter
2018-12-17 8:03 ` Oleksandr Andrushchenko
2018-12-18 19:20 ` Noralf Trønnes
2018-12-19 8:18 ` Oleksandr Andrushchenko
2018-12-19 13:14 ` Gerd Hoffmann
2018-12-19 13:21 ` Oleksandr Andrushchenko
2018-12-19 14:10 ` Gerd Hoffmann
2018-12-20 11:19 ` Oleksandr Andrushchenko [this message]
2018-12-20 15:37 ` Christoph Hellwig
2018-12-19 16:14 ` Noralf Trønnes
2018-12-20 11:24 ` Oleksandr Andrushchenko
2018-12-20 15:36 ` Christoph Hellwig
2018-12-20 15:49 ` Oleksandr Andrushchenko
2018-12-20 17:39 ` Christoph Hellwig
2018-12-20 18:29 ` Daniel Vetter
2018-12-20 18:33 ` Christoph Hellwig
2018-12-20 18:35 ` Daniel Vetter
2018-12-20 18:38 ` Christoph Hellwig
2018-12-21 9:16 ` Oleksandr Andrushchenko
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