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Re: [Xen-devel] [PATCH 2/2] xen: merge temporary vcpu pinning scenarios
- To: Juergen Gross <JGross@xxxxxxxx>
- From: Jan Beulich <JBeulich@xxxxxxxx>
- Date: Tue, 23 Jul 2019 15:55:34 +0000
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- Cc: Tim Deegan <tim@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, IanJackson <ian.jackson@xxxxxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Tue, 23 Jul 2019 15:59:24 +0000
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- Thread-topic: [Xen-devel] [PATCH 2/2] xen: merge temporary vcpu pinning scenarios
On 23.07.2019 17:22, Juergen Gross wrote:
> On 23.07.19 17:04, Jan Beulich wrote:
>> On 23.07.2019 16:29, Juergen Gross wrote:
>>> On 23.07.19 16:14, Jan Beulich wrote:
>>>> Oh, no. The #MC side use has gone away in 3a91769d6e, without cleaning
>>>> up other code. So there doesn't seem to be any such requirement anymore.
>>>
>>> So just to be sure: you are fine for me removing the pinning for NMIs?
>>
>> No, not the pinning as a whole. The forced CPU0 affinity should still
>> remain. It's just that there's no correlation anymore between the CPU
>> a vCPU was running on and the CPU it is to be pinned to (temporarily).
>
> I don't get it. Today vcpu0 of the hardware domain is pinned to the cpu
> it was last running on when the NMI happened. Why is that important?
> Or do you want to change the logic and pin vcpu0 for NMI handling always
> to CPU0?
Hmm, I've been confused. To me it doesn't make sense to force
affinity other than to CPU0 for an NMI. Hence I was mis-reading
the original send_guest_trap() invocation (in particular its
middle argument). Looking at commit 355b0469a8 ("x86: MCA support")
I would guess the affinity change for NMI delivery was done by
mistake, or at best just to not distinguish NMI from #MC.
As a result, considering that before that there was no affinity
change at all, and considering further than changing to other
than CPU0 doesn't really make sense, but we got away with not
doing so for so many years, I think I agree that it can be ripped
out altogether. Personally I'd prefer a fix to make it use CPU0,
but I'm in no way going to insist.
Jan
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