Re: [Xen-devel] [PATCH V2] 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 21:08, 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.
> 
> 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. We must ensure that 
> the
> IRET from the INT 2 IRET is the first IRET issued to prevent losing an NMI.
> Moving the INT 2 call to before the interrupts are enabled should ensure we
> don't lose the NMI.
> 
> Signed-off-by: Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>
> Acked-by: Tim Deegan <tim@xxxxxxx>

As (I think) we agreed to not use "int $2", and as I don't recall
having seen a v3 of this patch - is that something that can be
expected any time soon? Ideally, I would want to incorporate
the changes here (and hopefully also the PV issue described
during the discussion) in the pending 4.2.1 (and, if applicable,
4.1.4) release(s).

Jan

> diff -r 62885b3c34c8 -r 7d6fd0219dd7 xen/arch/x86/hvm/vmx/vmx.c
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -2269,6 +2269,14 @@ void vmx_vmexit_handler(struct cpu_user_
>          vector = intr_info & INTR_INFO_VECTOR_MASK;
>          if ( vector == TRAP_machine_check )
>              do_machine_check(regs);
> +        else if ( vector == TRAP_nmi &&
> +                ( (intr_info & INTR_INFO_INTR_TYPE_MASK) ==
> +                  (X86_EVENTTYPE_NMI << 8) ) )
> +            /* Must be called before interrupts are enabled to ensure
> +             * the NMI handler code is run before the first IRET. The
> +             * IRET unblocks subsequent NMI's (Intel SDM Vol 3, 6.7.1)
> +             */
> +            asm volatile("int $2"); /* Real NMI, vector 2: normal 
> processing */
>          break;
>      case EXIT_REASON_MCE_DURING_VMENTRY:
>          do_machine_check(regs);
> @@ -2442,7 +2450,6 @@ 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. */
>              break;
>          case TRAP_machine_check:
>              HVMTRACE_0D(MCE);
> 
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> http://lists.xen.org/xen-devel 




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