Re: Overlaped PIO with multiple ioreq_server (Xen4.6.1)

[Paolo Bonzini <pbonzini@redhat.com>]

On 28/04/2016 13:25, Paul Durrant wrote:
>> Maybe you are lucky, qemu is registered before your own demu
>> emulator.
> 
> I guess I was lucky.

Yeah, QEMU has been doing that since 2013 (commit 3bb28b7, "memory:
Provide separate handling of unassigned io ports accesses", 2013-09-05).

>> I used for testing your "demu" 2 years ago, now extending Citrix
>> "vgpu", all was fine up to xen 4.5.2 (with qemu 2.0.2) but
>> problem begin when I switched to 4.6.1 (with qemu 2.2.1), but it
>> maybe lucky timing in registration.
> 
> I think Xen should really be spotting range overlaps like this, but
> the QEMU<->Xen interface will clearly need to be fixed to avoid the
> over-claiming of I/O ports like this.

If the handling of unassigned I/O ports is sane in Xen (in QEMU they
return all ones and discard writes), it would be okay to make the
background 0-65535 range conditional on !xen_enabled().  See
memory_map_init() in QEMU's exec.c file.

Paolo

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