Re: [Qemu-devel] [PATCH 0/3] vga: add endianness switching support

[Benjamin Herrenschmidt]

On Tue, 2014-09-23 at 14:30 +0200, Gerd Hoffmann wrote:
>   Hi,

(Adding Mike)

> Series applies on top of the vga patches posted yesterday.
> It adds a mmio register to the pci stdvga cards allowing
> to switch framebuffer endiannness.  Ben's patches 1+2 prepare
> for runtime-swicthable endianness and patch #3 actually
> implements the new register.  It has been placed in a new
> range to avoid conflicts in case the bochs dispi interface
> will be extended in the future.  The separate range also
> makes it easier to turn off on old machine types.

Thanks. I'll try to produce patches for offb and bochsdrmfb to
adjust endianness & test some time this week or next week.

Now the remaining point of contention is the "auto switch"
functionality which, sadly, is needed for existing LE guests to work
properly.

I understand the reticence, on the other hand, our "pseries" machine is
really a paravirtualized environment where qemu implements the
hypervisor hcall interface, so in that context, it does make some sense
that the "hypervisor" affects some HW configuration/state on endian
switch (which is such an hcall).

The problem is how to do that in a reasonably clean way from an
implementation perspective.

The patch at
https://github.com/ozbenh/qemu/commit/778c92cebcd12fdafd2050dc4d7ca2521c462e46 
is one attempt to do it, for whatever it's worth, but I'm open to alternatives.

Cheers,
Ben.