Re: [Qemu-devel] [RFC] qemu VGA endian swap low level drawing changes

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2014-06-17 at 23:12 +0100, Peter Maydell wrote:
> On 17 June 2014 22:32, Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
> > Additionally, I wouldn't mind of we did a quick "trick" equivalent (but
> > cleaner) to what I did in my patch which is when the pseries guest calls
> > the hypervisor call to change the interrupt endian mode, we notify VGA
> > and switch its endian mode, so we work "by default" with kernels not
> > updated to know about that register. But this is open for debate. It's
> > somewhat "acceptable" in the context of our hypercall being a
> > "paravirtualized" interface, so it can be argued that the hypercall
> > poking at the VGA chip is equivalent to some FW doing so :-)
> 
> I'm definitely against this. Have your guest change the behaviour
> of the VGA device by explicitly prodding the VGA device, not by
> back-door side-effects of something else that it happens to do
> on one particular guest for one particular architecture, please.

But that means modifying guests ... obviously you live in a world where
you don't have to live with existing enterprise distros backward
compatibility :-)

I understand the reluctance against backdoor side effects in general,
but as I said, this is a hypervisor call that basically says "change
system endianness". It does make some amount of sense to have in that
case the hypervisor (which here is qemu) go adjust the endianness
setting in some devices as well as the cores.

Cheers,
Ben.