From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Subject: Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking
Date: Mon, 05 Sep 2011 15:05:01 +0300
Message-ID: <4E64BAED.2040101@redhat.com>
References: <cover.1314922370.git.jeremy.fitzhardinge@citrix.com>   <38bb37e15f6e5056d5238adac945bc1837a996ec.1314922370.git.jeremy.fitzhardinge@citrix.com>  <1314974826.1861.1.camel@twins> <4E612EA1.20007@goop.org> <1314996468.8255.0.camel@twins> <4E614FBD.2030509@goop.org>
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To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Peter Zijlstra <peterz@infradead.org>, "H. Peter Anvin" <hpa@zytor.com>, Linus Torvalds <torvalds@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>, the arch/x86 maintainers <x86@kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Nick Piggin <npiggin@kernel.dk>, Marcelo Tosatti <mtosatti@redhat.com>, KVM <kvm@vger.kernel.org>, Andi Kleen <andi@firstfloor.org>, Xen Devel <xen-devel@lists.xensource.com>, Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>
List-Id: xen-devel@lists.xenproject.org

On 09/03/2011 12:50 AM, Jeremy Fitzhardinge wrote:
> On 09/02/2011 01:47 PM, Peter Zijlstra wrote:
> >  On Fri, 2011-09-02 at 12:29 -0700, Jeremy Fitzhardinge wrote:
> >>>  I know that its generally considered bad form, but there's at least one
> >>>  spinlock that's only taken from NMI context and thus hasn't got any
> >>>  deadlock potential.
> >>  Which one?
> >  arch/x86/kernel/traps.c:nmi_reason_lock
> >
> >  It serializes NMI access to the NMI reason port across CPUs.
>
> Ah, OK.  Well, that will never happen in a PV Xen guest.  But PV
> ticketlocks are equally applicable to an HVM Xen domain (and KVM guest),
> so I guess there's at least some chance there could be a virtual
> emulated NMI.  Maybe?  Does qemu do that kind of thing?

kvm does.  In fact, I want to use 'hlt' for blocking vcpus in the 
slowpath and an NMI IPI for waking them up.  That's not going to work in 
an NMI, but I guess I can replace the 'hlt' with a 'pause' if we're in 
an NMI, and just spin there.

btw, doing a race-free NMI wakeup is going to be interesting - we'll 
have to look at %rip and see if is just before our hlt, since there's no 
magic sti; hlt sequence we can use.

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