Re: [Xen-devel] [PATCH] x86/vlapic: Don't reset APIC mode/ID when handling INIT signal

[Juergen Gross <jgross@xxxxxxxx>]

On 19/04/17 15:43, Boris Ostrovsky wrote:
> 
> 
> On 04/19/2017 09:21 AM, Jan Beulich wrote:
>>>>> On 19.04.17 at 06:56, <chao.gao@xxxxxxxxx> wrote:
>>> On Wed, Apr 19, 2017 at 02:48:57AM -0600, Jan Beulich wrote:
>>>>>>> On 18.04.17 at 23:51, <chao.gao@xxxxxxxxx> wrote:
>>>>> HVM guest can't enable x2apic as XENFEAT_hvm_pirqs is exposed to it.
>>>>> Why we have this restriction? As a consequence, This patch isn't
>>>>> tested in that case.
>>>>
>>>> HVM guests very well can use x2APIC, I'm seeing them use it all
>>>> the time. Please be more specific with your question.
>>>
>>> OK. I failed to enable guest's x2apic, with the output: IRQ Remapping
>>> doesn't
>>> support X2APIC mode. Eventually, I found the xen_x2apic_para_available()
>>> in linux kernel and it returns false when Xen exposes
>>> XENFEAT_hvm_pirqs to
>>> guest.
>>
>> Looks more like a Linux/pvops question, which I continue to not
>> really be an expert on. Let's ask the maintainers (now Cc-ed)
>> (but it may well be that this simply depends on whether you
>> enable PVHVM mode)...
> 
> Interestingly enough, I was looking at this last week and wondering why
> it was there. Ankur (copied) found an issue related to x2apic he thought
> was PVH-specific, but that was because HVM guests don't turn it on.
> 
> Konrad couldn't remember why it was there neither.

I think this is the answer:

https://lists.xen.org/archives/html/xen-devel/2010-12/msg00373.html

Basically some very old assumption...

I think we can remove the offending test and do some more cleanup in
this area (I think there are some #ifdef's and tests which can be
removed, too).

I'll post some patches.


Juergen


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