Re: [Xen-devel] x86/vMSI-X emulation issue

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 24 March 2016 09:35
> To: Paul Durrant
> Cc: Andrew Cooper; xen-devel
> Subject: RE: [Xen-devel] x86/vMSI-X emulation issue
> 
> >>> On 24.03.16 at 10:09, <Paul.Durrant@xxxxxxxxxx> wrote:
> >> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxx] On Behalf Of
> Jan
> >> Beulich
> >> Sent: 24 March 2016 07:52
> >> > 2) Do aforementioned chopping automatically on seeing
> >> >     X86EMUL_UNHANDLEABLE, on the basis that the .check
> >> >     handler had indicated that the full range was acceptable. That
> >> >     would at once cover other similarly undesirable cases like the
> >> >     vLAPIC code returning this error. However, any stdvga like
> >> >     emulated device would clearly not want such to happen, and
> >> >     would instead prefer the entire batch to get forwarded in one
> >> >     go (stdvga itself sits on a different path). Otoh, with the
> >> >     devices we have currently, this would seem to be the least
> >> >     intrusive solution.
> >>
> >> Having thought about it more over night, I think this indeed is
> >> the most reasonable route, not just because it's least intrusive:
> >> For non-buffered internally handled I/O requests, no good can
> >> come from forwarding full batches to qemu, when the respective
> >> range checking function has indicated that this is an acceptable
> >> request. And in fact neither vHPET not vIO-APIC code generate
> >> X86EMUL_UNHANDLEABLE. And vLAPIC code doing so is also
> >> just apparently so - I'll submit a patch to make this obvious once
> >> tested.
> >>
> >> Otoh stdvga_intercept_pio() uses X86EMUL_UNHANDLEABLE in
> >> a manner similar to the vMSI-X code - for internal caching and
> >> then forwarding to qemu. Clearly that is also broken for
> >> REP OUTS, and hence a similar rep count reduction is going to
> >> be needed for the port I/O case.
> >
> > It suggests that such cache-and/or-forward models should probably sit
> > somewhere else in the flow, possibly being invoked from
> hvm_send_ioreq()
> > since there should indeed be a selected ioreq server for these cases.
> 
> I don't really think so. As I have gone through and carried out
> what I had described above, I think I managed to address at
> least one more issue with not properly handled rep counts, and
> hence I think doing it that way is correct. I'll have to test the
> thing before I can send it out, for you to take a look.
> 

Ok. I never particularly liked using X86EMUL_UNHANDLEABLE to invoke the 
forwarding behaviour though as it's only legitimate to do it on the first rep. 
I always had the feeling there had to be a nicer way of doing it. Possibly just 
too intrusive a change at this point though.

  Paul

> Jan


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