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Re: [Xen-devel] [PATCH V3] vmx/nmi: Do not use self_nmi() in VMEXIT handler



>>> On 22.11.12 at 16:00, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> The self_nmi() code cause's an NMI to be triggered by sending an APIC
> message to the local processor.  However, NMIs are blocked by the
> VMEXIT, until the next iret or VMENTER.
> 
> 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.
> 
> As a result, as soon as the VMENTER happens, an immediate VMEXIT
> happens as a result of the queued NMI.  We have seen hundreds of
> iterations of this VMEXIT/VMENTER loop before the HVM guest resumes
> normal operation.
> 
> Signed-off-by: Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> 
> --
> Changes since v2
>  * Switch from 'int $2' to do_nmi()
>  * Reworked commit message to more clearly explain the problem
> 
> diff -r 2489c2926698 -r d7ea938044ac 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)
> +             */
> +            do_nmi();

But that's only half of it, at least as far as I recall the outcome of
the discussion: You want an IRET (or VMRESUME) before possibly
going into the scheduler (and hence not back to the current VM).
And the same also on the PV exit path from an NMI.

Jan

>          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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