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Date: Mon, 5 Sep 2011 12:52:42 +0100 From: Stefano Stabellini <stefano.stabellini@...citrix.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>, Avi Kivity <avi@...hat.com>, 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>, Keir Fraser <keir@....org> Subject: Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking 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 linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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