----- Original Message -----
> From: "David Hildenbrand" <da...@redhat.com>
> To: "Paolo Bonzini" <pbonz...@redhat.com>, "Pavel Dovgalyuk" 
> <pavel.dovga...@ispras.ru>, qemu-devel@nongnu.org
> Cc: kw...@redhat.com, "peter maydell" <peter.mayd...@linaro.org>, "boost 
> lists" <boost.li...@gmail.com>,
> quint...@redhat.com, jasow...@redhat.com, m...@redhat.com, 
> zuban...@gmail.com, "maria klimushenkova"
> <maria.klimushenk...@ispras.ru>, dovga...@ispras.ru, kra...@redhat.com, "alex 
> bennee" <alex.ben...@linaro.org>
> Sent: Thursday, November 2, 2017 7:39:22 PM
> Subject: Re: [RFC PATCH 13/26] cpus: only take BQL for sleeping threads
> 
> On 02.11.2017 12:08, Paolo Bonzini wrote:
> > On 31/10/2017 12:26, Pavel Dovgalyuk wrote:
> >> From: Alex Bennée <alex.ben...@linaro.org>
> >>
> >> Now the only real need to hold the BQL is for when we sleep on the
> >> cpu->halt conditional. The lock is actually dropped while the thread
> >> sleeps so the actual window for contention is pretty small. This also
> >> means we can remove the special case hack for exclusive work and
> >> simply declare that work no longer has an implicit BQL held. This
> >> isn't a major problem async work is generally only changing things in
> >> the context of its own vCPU. If it needs to work across vCPUs it
> >> should be using the exclusive mechanism or possibly taking the lock
> >> itself.
> >>
> >> Signed-off-by: Alex Bennée <alex.ben...@linaro.org>
> >> Tested-by: Pavel Dovgalyuk <pavel.dovga...@ispras.ru>
> > 
> > At least cpu_throttle_thread would fail with this patch.
> > 
> > Also I am not sure if the s390 SIGP handlers are ready for this.
> > 
> 
> We have a global lock to the SIGP "facility". However we need the BQL in
> order to inject interrupts into CPUs (otherwise it would trigger an
> assert when injecting).
> 
> We inject Restart and Stop interrupts from run_on_cpu. This requires the
> BQL. So Paolo should be right, this change would break s390x.

I had some patches to access interrupt_request with the atomic builtins.  If
Pavel can first extract the other changes to the icount mechanism, I can
update them.

Paolo

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