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

[Gerd Hoffmann]

On Thu, Mar 24, 2022 at 10:20:40AM +0100, Christian König wrote:
> Hi Cong,
> 
> when I understand Robin correctly all mapping (host, guest, kernel,
> userspace etc..) must have the same caching attributes unless you use the
> S2FWB feature introduced with Armv8.4.
> 
> If you don't follow those rules you usually run into coherency issues or
> even worse system hangs. So you not only need to adjust the kernel mappings,
> but also the function for userspace mappings to follow those rules.

That matches my understanding.

For qxl specifically: when using the xork qxl driver getting the
userspace mappings right is essential because userspace will write qxl
command buffers then.  When using the xorg modesetting driver or wayland
the worst thing happening would be display corruption because userspace
will only map dumb bo's for pixel data.

I'm wondering though why you are keen on getting qxl work instead of
just using virtio-gpu?

take care,
  Gerd