Re: [Xen-devel] [PATCH V2] xen: vmx: Use an INT 2 call to process real NMI's instead of self_nmi() in VMEXIT handler

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/11/12 08:58, Jan Beulich wrote:
>>>> 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

Malcolm is out of the office this week.  I will see about respinning a
v3 later today.

~Andrew

>
>> 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);
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxx 
>> http://lists.xen.org/xen-devel 
>
>
>
> _______________________________________________
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> Xen-devel@xxxxxxxxxxxxx
> http://lists.xen.org/xen-devel

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com


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