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. */ -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
