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Re: [Xen-devel] Interrupt to CPU routing in HVM domains - again

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Interrupt to CPU routing in HVM domains - again
From: John Levon <levon@xxxxxxxxxxxxxxxxx>
Date: Fri, 5 Sep 2008 02:21:08 +0100
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, bart brooks <bart_brooks@xxxxxxxxxxx>
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On Fri, Sep 05, 2008 at 11:06:06AM +1000, James Harper wrote:

> As per a recent discussion I modified the IRQ code in the Windows GPLPV
> drivers so that only the vcpu_info[0] structure is used, instead of
> vcpu_info[current_cpu] structure. As per Bart's email below though, this
> has caused him to experience performance issues.
> Have I understood correctly that only cpu 0 of the vcpu_info[] array is
> ever used even if the interrupt actually occurs on another vcpu? Is this
> true for all versions of Xen? It seems that Bart's experience is exactly
> the opposite of mine - the change that fixed up the performance issues
> for me caused performance issues for him...

I was just in this code. From what I can tell it's a requirement that
all the domU's event channels be bound to VCPU0, since there's no code
in Xen itself that will redirect evtchn_pending_sel from another VCPU to
CPU0's vcpu_info, and the callback IRQ code assumes VCPU0.


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