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Re: [Xen-devel] INIT-SIPI-SIPI timing and CPU bringup on new Intel CPUs

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Subject: Re: [Xen-devel] INIT-SIPI-SIPI timing and CPU bringup on new Intel CPUs
From: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Date: Tue, 19 Jul 2011 13:37:51 +0100
Cc: "Lv, Hui" <hui.lv@xxxxxxxxx>
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At 16:31 +0100 on 23 Jun (1308846702), Tim Deegan wrote:
> I've found that removing the delays between the IPIs makes this problem
> go away.  I'm hoping someone from Intel can help me understand why, and
> whether that's a sensible answer.  Since this is an x2apic system I
> guess we don't have to wait for the IPI to be acked by the sending APIC,
> but the SDMs still recommend delaying between INIT, SIPI and SIPI, and I
> can imagine I might be breaking some other systems by doing this.
> So the questions I have are:
>  - is it safe always to issue INIT-SIPI-SIPI back-to-back on x2apic systems?
>  - is there any reason why the delay should make a difference (except 
>    maybe that this is an early version of a chip?)
> My patch is attached; it removes the delays only in the x2apic case,
> so shouldn't break any older systems.

We've tested this on the XenServer automated test pools for a couple of
weeks and not seen any ill effects.  In the absence of any negative
feedback, I propose that we apply this patch to xen-unstable.



Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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