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

[Tim Deegan <tim@xxxxxxx>]

At 17:33 +0000 on 15 Nov (1353000782), Mats Petersson wrote:
> On 15/11/12 17:15, Tim Deegan wrote:
> >At 17:03 +0000 on 15 Nov (1352998993), Mats Petersson wrote:
> >>>On an AMD CPU we _don't_ have dedicated stacks for NMI or MCE when we're
> >>>running a HVM guest, so the stack issue doesn't apply (but nested NMIs
> >>>are still bad).
> >>>
> >>>On an Intel CPU, we _do_ use dedicated stacks for NMI and MCE in HVM
> >>>guests.  We don't really have to but it saves time in the context switch
> >>>not to update the IDT.  Using do_nmi() here means that the first NMI is
> >>>handled on the normal stack instead.  It's also consistent with the way
> >>>we call do_machine_check() for the MCE case.  But it needs an explicit
> >>>IRET after the call to do_nmi() to make sure that NMIs get re-enabled.
> >>Both AMD and Intel has an identical setup with regard to stacks and
> >>general "what happens when we taken one of these interrupts".
> >My reading of svm_ctxt_switch_{to,from} makes me disagree with this.
> >AFAICT, on SVM we're not using dedicated stacks at all.
> In SVM, the VMRUN returns to whatever stack you had before the VMRUN. 
> This is not what I'm talking about, however. The stack used for the NMI 
> and MCE comes from the interrupt descriptor entry for those respective 
> vectors.

This is the code I was referring to:

    /*
     * Cannot use ISTs for NMI/#MC/#DF while we are running with the guest TR.
     * But this doesn't matter: the IST is only req'd to handle SYSCALL/SYSRET.
     */
    idt_tables[cpu][TRAP_double_fault].a  &= ~(7UL << 32);
    idt_tables[cpu][TRAP_nmi].a           &= ~(7UL << 32);
    idt_tables[cpu][TRAP_machine_check].a &= ~(7UL << 32);

Am I misreading it?

> So in conclusion, the do_mce_exception() call probably should be a 
> __asm__ __volatile__("int $X"), where X is the relevant vector. 

This handles MCEs that were raised in guest context.  If we've managed
to get this far into the exit handler, the hypervisor stack is probably
OK. :)

I'd be happy to invoke the MCE handler though the IDT here, just for
symmetry with the other cases, but I don't think it makes much
difference. 

Tim.

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