Re: [Xen-devel] XenGT is still regressed on master

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 08/03/2019 16:14, Jan Beulich wrote:
>>>> On 08.03.19 at 16:18, <igor.druzhinin@xxxxxxxxxx> wrote:
>> On 08/03/2019 14:58, Jan Beulich wrote:
>>>>>> On 08.03.19 at 15:25, <igor.druzhinin@xxxxxxxxxx> wrote:
>>>> On 08/03/2019 11:55, Jan Beulich wrote:
>>>>
>>>> I like the latter suggestion more. Seems less invasive and prone to
>>>> regressions. I'd like to try to implement it. Although I think the
>>>> hypervisor check should be more general: like if IOREQ is in progress
>>>> don't try to got through fast-path and re-enter IOREQ completion path.
>>>>
>>>> What if we just check !hvm_ioreq_needs_completion() before returning
>>>> X86EMUL_OKAY i.e. fall through to the bad_gfn_to_mfn case if that check
>>>> fails as Paul suggested?
>>>
>>> I didn't see such a suggestion, I think, and I'm afraid it would still not
>>> be correct in the general case. As said before, Getting back
>>> HVMTRANS_okay means the write did actually complete, and no
>>> second attempt to do the write should be done.
>>
>> What if we don't do hvm_copy() in that case and will go to slow-path
>> directly, would that be better?
> 
> Ah yes, that looks like a better approach (provided Paul also gives it
> his okay). There being an ioreq in flight is a fair indication that we will
> want to enter hvmemul_linear_mmio_{read,write}().
> 
> One caveat though: What do you suggest to do with page straddling
> accesses? The commit introducing these functions was, after all to
> deal with this special case. The in-flight request we observe there
> could be for the leading or trailing part of the access that's being
> re-executed. While these could perhaps be told apart by looking at
> the low address bits, similarly what do you do for multi-part (but
> perhaps well aligned) accesses like cmps*, vgather*, or vscatter*?

I don't think there is a problem here: IOREQs are issued sequentially
for each part of the access. hvmemul_linear_mmio_access() makes sure one
chunk of the access isn't straddling a page boundary and we're finishing
the requested part immediately after an IOREQ for it got issued. I don't
see how it could enter linear_{read,write}() for a wrong part unless the
emulation layer above is broken.

Igor

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