Re: [Xen-devel] [PATCH] libxl: Remove qxl support for the 4.3 release

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Wed, May 29, 2013 at 11:25 PM, Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
> On 29/05/2013 08:43, Ian Campbell wrote:
>> On Tue, 2013-05-28 at 19:09 +0100, Keir Fraser wrote:
>>> On 28/05/2013 17:51, "Ian Jackson" <Ian.Jackson@xxxxxxxxxxxxx> wrote:
>>>
>>>> George Dunlap writes ("[PATCH] libxl: Remove qxl support for the 4.3
>>>> release"):
>>>>> The qxl drivers for Windows and Linux end up calling instructions
>>>>> that cannot be used for MMIO at the moment.  Just for the 4.3 release,
>>>>> remove qxl support.
>>>>>
>>>>> This patch should be reverted as soon as the 4.4 development window opens.
>>>>>
>>>>> The issue in question:
>>>>>
>>>>> (XEN) emulate.c:88:d18 bad mmio size 16
>>>>> (XEN) io.c:201:d18 MMIO emulation failed @ 0033:7fd2de390430: f3 0f 6f
>>>>> 19 41 83 e8 403
>>>>>
>>>>> The instruction in question is "movdqu (%rcx),%xmm3".  Xen knows how
>>>>> to emulate it, but unfortunately %xmm3 is 16 bytes long, and the interface
>>>>> between Xen and qemu at the moment would appear to only allow MMIO 
>>>>> accesses
>>>>> of 8 bytes.
>>>>>
>>>>> It's too late in the release cycle to find a fix or a workaround.
>>>> Acked-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
>>> It could be plumbed through hvmemul_do_io's multi-cycle read/write logic,
>>> and done as two 8-byte cycles to qemu. This would avoid bloating the ioreq
>>> structure that communicates to qemu.
>> Are you proposing we do this for 4.3? I'm not sure how big that change
>> would be in terms of impact (just that one instruction, any 16 byte
>> operand?).
>>
>> Of course even if we did this for 4.3 we don't know what the next issue
>> will be with QXL.
>>
>> Ian.
>
> Furthermore, AVX instruction emulation would require support for 32byte
> operands.  I don't see the multi-cycle logic scaling sensibly.

Andrew, Keir, Jan, does any one of you fancy taking this on for 4.4?

 -George

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