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Re: [Qemu-devel] [PATCH v2 9/9] target-arm: Add WFx instruction trap sup


From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH v2 9/9] target-arm: Add WFx instruction trap support
Date: Thu, 23 Apr 2015 15:30:56 +0100

On 23 April 2015 at 15:26, Greg Bellows <address@hidden> wrote:
> Good catch Edgar, we shouldn't trap if we are "going into a low-power
> state".

You mean "if we are not going into a low-power state".

> It sounds like we arrived at the following:
>
> wfi-  check has_work before taking either the UDEF or HLT exception hereby
> making it a noop.

This is OK (as per the code sketch in my earlier message).

> wfe - Never trap, making it always a noop.  This includes undoing the
> existing YIELD exception.

No, the YIELD behaviour is correct for WFE. This is important
to avoid us spinning for the rest of this vcpu's timeslice
(never making progress because the other vcpu which could
unblock us from this spinlock doesn't run). But we don't ever
want to trap to EL2.

-- PMM



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