linux-kernel - Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking

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Message-ID: <4E64BAED.2040101@redhat.com>
Date:	Mon, 05 Sep 2011 15:05:01 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>, Andi Kleen <andi@...stfloor.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>
Subject: Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking

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

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