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

[Alexander Graf <agraf@suse.de>]

On 17.06.14 11:18, Alexey Kardashevskiy 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.
>
> Ideas?

The register approach is nicer. Let's wait for Gerd to see what he thinks.


Alex