From: Robin Murphy <robin.murphy@arm.com>
To: Gerd Hoffmann <kraxel@redhat.com>
Cc: airlied@linux.ie, dri-devel@lists.freedesktop.org,
virtualization@lists.linux-foundation.org, ray.huang@amd.com,
"Cong Liu" <liucong2@kylinos.cn>,
daniel@ffwll.ch, spice-devel@lists.freedesktop.org,
airlied@redhat.com, "Christian König" <christian.koenig@amd.com>
Subject: Re: [PATCH v1 1/2] drm/qxl: replace ioremap by ioremap_cache on arm64
Date: Wed, 23 Mar 2022 10:26:52 +0000 [thread overview]
Message-ID: <cfd7b57e-0573-db04-3692-acedd20fb51e@arm.com> (raw)
In-Reply-To: <20220323101138.7oiwefh7ofcxojgq@sirius.home.kraxel.org>
On 2022-03-23 10:11, Gerd Hoffmann wrote:
> On Wed, Mar 23, 2022 at 09:45:13AM +0000, Robin Murphy wrote:
>> On 2022-03-23 07:15, Christian K�nig wrote:
>>> Am 22.03.22 um 10:34 schrieb Cong Liu:
>>>> qxl use ioremap to map ram_header and rom, in the arm64 implementation,
>>>> the device is mapped as DEVICE_nGnRE, it can not support unaligned
>>>> access.
>>>
>>> Well that some ARM boards doesn't allow unaligned access to MMIO space
>>> is a well known bug of those ARM boards.
>>>
>>> So as far as I know this is a hardware bug you are trying to workaround
>>> here and I'm not 100% sure that this is correct.
>>
>> No, this one's not a bug. The Device memory type used for iomem mappings is
>> *architecturally* defined to enforce properties like aligned accesses, no
>> speculation, no reordering, etc. If something wants to be treated more like
>> RAM than actual MMIO registers, then ioremap_wc() or ioremap_cache() is the
>> appropriate thing to do in general (with the former being a bit more
>> portable according to Documentation/driver-api/device-io.rst).
>
> Well, qxl is a virtual device, so it *is* ram.
>
> I'm wondering whenever qxl actually works on arm? As far I know all
> virtual display devices with (virtual) pci memory bars for vram do not
> work on arm due to the guest mapping vram as io memory and the host
> mapping vram as normal ram and the mapping attribute mismatch causes
> caching troubles (only noticeable on real arm hardware, not in
> emulation). Did something change here recently?
Indeed, Armv8.4 introduced the S2FWB feature to cope with situations
like this - essentially it allows the hypervisor to share RAM-backed
pages with the guest without losing coherency regardless of how the
guest maps them.
Robin.
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prev parent reply other threads:[~2022-03-23 10:27 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20220322093444.1236582-1-liucong2@kylinos.cn>
2022-03-23 7:15 ` [PATCH v1 1/2] drm/qxl: replace ioremap by ioremap_cache on arm64 Christian König via Virtualization
2022-03-23 9:45 ` Robin Murphy
2022-03-23 9:46 ` Christian König via Virtualization
2022-03-23 10:11 ` Gerd Hoffmann
2022-03-23 10:26 ` Robin Murphy [this message]
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