Re: [PATCH 3 of 4] VIOAPIC: Emulate a version 0x20 IOAPIC

[Keir Fraser <keir.xen@gmail.com>]

On 08/02/2012 17:05, "Tim Deegan" <tim@xen.org> wrote:

> At 16:45 +0000 on 08 Feb (1328719538), Andrew Cooper wrote:
>> Currently, hvm emulates a version 0x11 IOAPIC.  However, depending on
>> the HVM guests {IO,L}APIC setup, it may take fewer traps into Xen by
>> directly using the VIOAPIC EOI register present in version 0x20,
>> rather than resorting to the legacy method of flipping the trigger
>> mode for the relevent RTE.
>> 
>> Currently, all required functionality is already present in the code,
>> except that it is covered by VIOAPIC_IS_IOSAPIC which is never defined.
>> 
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> 
> This probably ought to introduce a hvm save record to say which kind of
> IOAPIC the VM was booted with, so that after a live migration the OS
> doesn't get confused. :(
> 
> It seems unlikely that any OSes rely on the IOAPIC version (and the
> behaviour of that register) being static after boot, but better safe
> than sorry - there might be some confusion in resume from S3 or similar.

Yes, so unless there is actually a demonstrable win from bumping the
version, we should just not bother, and cull the IS_IOSAPIC code sections.

 -- Keir