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

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Subject: [Xen-devel] INIT-SIPI-SIPI timing and CPU bringup on new Intel CPUs
From: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Date: Thu, 23 Jun 2011 16:31:42 +0100
Cc: "Lv, Hui" <hui.lv@xxxxxxxxx>
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I've come across the problem reported in this thread:
where on a very new Intel system some of the APs don't respond when the
BSP tries to wake them.

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.



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

Attachment: sipi-timing.patch
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