[Xen-devel] Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking

[Avi Kivity <avi@xxxxxxxxxx>]

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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