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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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

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

Ian.


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