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Date: Thu, 10 May 2012 12:35:26 +0300
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1.1] Switch SIG_IPI to SIGUSR1
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To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Peter Maydell <peter.maydell@linaro.org>, Anthony Liguori <aliguori@us.ibm.com>, Michael Tokarev <mjt@tls.msk.ru>, qemu-devel <qemu-devel@nongnu.org>

On 05/10/2012 01:14 AM, Jan Kiszka wrote:
> Use SIGUSR1 unconditionally as SIG_IPI. First, ucontext coroutines tend
> to corrupt RT signal masks due to a 32-on-64-bit Linux kernel bug. And,
> second, there appears to be no advantage in using RT signals for VCPU
> kicking.
>
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
>
> As people prefer to stick with ucontext, this replaces the backend
> switch for i386-linux.
>
> Avi, you once committed the RT signal based SIG_IPI version to
> qemu-kvm. Can you confirm that SIGUSR1 comes with no downside?
>

In fact SIGUSR1 would be better in that real-time signals are queued, so
two IPIs are delivered twice with the old SIG_IPI and only once with
SIGUSR1.

I have some vague recollection that real time signals are handled faster
in the kernel.  Even if this is true, it's meaningless with kernel
irqchip since signalling is only used heavily with userspace APIC.

-- 
error compiling committee.c: too many arguments to function