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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

CC'ing Keir.

On Fri, 2 Sep 2011, 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?

Xen knows how to inject NMIs to HVM guests, even though I am not sure
if it is actually done in practice at the moment.

However digging into the implementation details, it looks like virtual
NMIs are not injected if blocking-by-STI (or blocking-by-MOV-SS), so we
should be fine, even though I don't know if you actually want to rely on
this:

/*
 * We can only inject an NMI if no blocking by MOV SS (also, depending on
 * implementation, if no blocking by STI). If pin-based 'virtual NMIs'
 * control is specified then the NMI-blocking interruptibility flag is
 * also checked. The 'virtual NMI pending' control (available only in
 * conjunction with 'virtual NMIs') causes a VM exit when all these checks
 * succeed. It will exit immediately after VM entry if the checks succeed
 * at that point.
 * 
 * Because a processor may or may not check blocking-by-STI when injecting
 * a virtual NMI, it will be necessary to convert that to block-by-MOV-SS
 * before specifying the 'virtual NMI pending' control. Otherwise we could
 * enter an infinite loop where we check blocking-by-STI in software and
 * thus delay delivery of a virtual NMI, but the processor causes immediate
 * VM exit because it does not check blocking-by-STI.
 */

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