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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 13/11/12 12:05, Tim Deegan wrote:
> At 11:47 +0000 on 13 Nov (1352807234), Tim Deegan wrote:
>> At 11:38 +0000 on 13 Nov (1352806689), Jan Beulich wrote:
>>>> 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?
>> ... this is my doing.  There used to be a call to do_nmi() here, but
>> do_nmi() doesn't block NMIs, so you can't just call it here in case you
>> get _another_ NMI while you're in the NMI handler.
> Oh wait, I see -- you're saying that this (20059:76a65bf2aa4d) is wrong
> because NMIs are indeed blocked, and have been since the VMEXIT.
>
> In that case, I agree that we should just run the NMI handler, but first
> I would really like to know what _unblocks_ NMIs in this case.  Any of
> the things I can think of (the next vmenter, the next iret, ??) will
> need some handling to make sure they actually happen before, say, we
> take this CPU into the idle loop...
>
> Tim.

The first experiment was to have self_nmi() wait until the NMI count for
the current processor changed.  This prevented the thousands of bounces
in and out of non-root mode, but the enter delays fairly consistently in
the millions of TSC ticks (a few milliseconds on the test box), which is
a noticeable chunk of time.

~Andrew

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