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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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@eu.citrix.com> 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@eu.citrix.com>
>> 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

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