Re: [Xen-devel] [PATCH] xen: vmx: Use an INT 2 call to process real NMI's instead of self_nmi() in VMEXIT handler

[Tim Deegan <tim@xxxxxxx>]

At 13:29 +0000 on 13 Nov (1352813350), Ian Campbell wrote:
> On Tue, 2012-11-13 at 13:21 +0000, Malcolm Crossley wrote:
> > The self_nmi() code cause's an NMI to be triggered by sending an APIC 
> > message
> > to the local processor. Unfortunately there is a delay in the delivery of 
> > the
> > APIC message and we may already have re-entered the HVM guest by the time 
> > the
> > NMI is taken. This results in the VMEXIT code calling the self_nmi() 
> > function
> > again. We have seen hundreds of iterations of this VMEXIT/VMENTER loop 
> > before
> > the HVM guest resumes normal operation.
> > 
> > Volume 3 Chapter 27 Section 1 of the Intel SDM states:
> > 
> > An NMI causes subsequent NMIs to be blocked, but only after the VM exit
> > completes.
> > 
> > So we believe it is safe to directly invoke the INT call as NMI's should
> > already be blocked.
> > 
> > The INT 2 call will perform an IRET which will unblock later calls to the 
> > NMI
> > handler, according to Intel SDM Volume 3 Chapter 6.7.1.
> > 
> > Signed-off-by: Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>
> > Acked-by: Tim Deegan <tim@xxxxxxx>
> > 
> > diff -r 62885b3c34c8 -r e1fbee58b25c xen/arch/x86/hvm/vmx/vmx.c
> > --- a/xen/arch/x86/hvm/vmx/vmx.c
> > +++ b/xen/arch/x86/hvm/vmx/vmx.c
> > @@ -2442,7 +2442,7 @@ void vmx_vmexit_handler(struct cpu_user_
> >                   (X86_EVENTTYPE_NMI << 8) )
> >                  goto exit_and_crash;
> >              HVMTRACE_0D(NMI);
> > -            self_nmi(); /* Real NMI, vector 2: normal processing. */
> > +            asm("int $2"); /* Real NMI, vector 2: normal processing. */
> 
>                asm volatile("...")
> 
> I think? Otherwise this could potentially get hoisted up

Good catch.  Hoisted would be fine, but it could also be entirely
discarded. :)

> Do we need any clobbers? Specifically I'm thinking of memory since
> taking an int2 ought to preserve register state.

I don't think so - nothing on this path depends on the actual behaviour
of the NMI handler.

Tim.

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