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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.11.12 at 12:23, Malcolm Crossley <malcolm.crossley@xxxxxxxxxx> 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.

But it does, eventually? I ask because ...

> 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.

... this blocking of NMIs should last until you enter the guest
again, so I would actually expect this to be an infinite loop ...

> So we believe it is safe to directly invoke the INT call as NMI's should
> already be blocked.

... and this not only be safe, but actually required.

> diff -r 62885b3c34c8 -r ea756059a8da 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. */

In any case - why can't you call do_nmi() directly from here?

Jan

>              break;
>          case TRAP_machine_check:
>              HVMTRACE_0D(MCE);
> 
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