[PATCH v1 1/2] drm/qxl: replace ioremap by ioremap_cache on arm64

[Robin Murphy]

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.