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SubjectRe: [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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