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

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Tue, 17 Jun 2014 19:18:11 +1000
Alexey Kardashevskiy <aik@ozlabs.ru> wrote:

> On 06/17/2014 03:36 PM, Paolo Bonzini wrote:
> > Il 17/06/2014 06:59, Benjamin Herrenschmidt ha scritto:
> >> Thanks. I've tried the other approach of adding new functions which
> >> means no overhead (hopefully) for the non-swap case and less invasive
> >> changes to vga_template.c.
> >>
> >> Patch below. What do you think ? This or the previous approach ? Then we
> >> can discuss how we actually trigger the endian change and where we store
> >> the state :-)
> > 
> > This is definitely more readable.  Anyway Gerd is the VGA guy. :)
> 
> I am that lucky person who got to do endianness-on-fly-switching
> QOM'fication :)
> 
> We have 2 ways of doing this:
> 
> 1. implement a VGA register in QEMU and use it from the guest;
> there is a try to discuss it in "[Qemu-devel] Endian control register";
> requires guest changes;
> 
> 2. on SPAPR we have H_SET_MODE hypercall which guest uses when it switches
> endianness and current hack is to change the flag directly in VGA device
> from this hypercall handler. Instead of that hack, we could have added a
> device callback:
> 
> void DeviceClass::endianness_notify(DeviceState *dev,
> 		enum device_endian endianness);
> 
> or even an Interface (with the same method alone). And in H_SET_MODE we
> could walk through all devices in the system and poke ones which implement
> a callback or an interface. virtio could benefit from this as well.
> 

There has been a discussion already about virtio endianness: relying on
a guest system wide setting such as LPCR_ILE has been strongly rejected
at the time... The consensus for virtio is "device endianness is the
endianness of the CPU that does the reset" (hence MSR_LE for PPC).

I don't know the details for VGA, but virtio would not benefit from
such callback.

> Ideas?
> 
> 



-- 
Gregory Kurz                                     kurzgreg@fr.ibm.com
                                                 gkurz@linux.vnet.ibm.com
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)562 165 496

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.