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[Xen-devel] RE: (v)MCE# injection

To: Jan Beulich <JBeulich@xxxxxxxxxx>, Christoph Egger <Christoph.Egger@xxxxxxx>, "Ke, Liping" <liping.ke@xxxxxxxxx>
Subject: [Xen-devel] RE: (v)MCE# injection
From: "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>
Date: Thu, 26 Nov 2009 17:55:47 +0800
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Thread-topic: (v)MCE# injection
Thanks for your checking very much. See comments below please.

Jan Beulich wrote:
> I'm having the impression that this is significantly flawed at
> the moment,
> and due to the number of issues I don't feel comfortable to submit a
> patch without clarification on at least some of the aspects:
> 1) The outermost conditional in do_iret() compares against
> VCPU_TRAP_NMI, which I would assume is a copy-and-paste mistake
> and should really be VCPU_TRAP_MCE.

Yes, it should be.

> 2) While c/s 19422 handled Dom0 only (and introduced some hardcoded
> references to dom0), c/s 19871 relaxed the check to permit any domain,
> but apparently failed to clean up the dom0 references.
> 3) I don't think the trap priority adjustment works properly at all,
> as there's only provision for a single level of nesting (due to
> the old value
> being stored in the vcpu structure), but the entry.S code would
> happily nest an MCE inside an NMI. I think these rather need to be -
> just like in
> physical CPUs - individual flags that tell whether an exception of
> that kind is currently being processed. And I don't think either
> should mask the other, the flags should just be used to prevent
> nested injection of the same type of exception (but there needs to be
> a way to tell which of the two was injected first, so the iret can
> clear the right flag). 

I remember Ke Liping and I discussed this before, but I forgot the conclusion. 
So I'd continue discussion here.

I'm not sure if it's always correct for the " don't think either should mask 
the other". Followed is quote from Intel's SDM section 6.9, PRIORITY AMONG 
SIMULTANEOUS EXCEPTIONS AND INTERRUPTS : "The processor first services a 
pending exception or interrupt from the class which has the highest priority, 
transferring execution to the first  instruction of the handler. Lower priority 
exceptions are discarded; lower priority interrupts are held pending. Discarded 
exceptions are re-generated when the interrupt handler returns execution to the 
point in the program or task where the exceptions and/or interrupts occurred.". 
Because this section is talking about "more than one exception or interrupt is 
pending at an instruction boundary", so I'm not sure how nested will happen. 
Can you share me where you get the idea of nested?

In fact, I suspect if we really need nested for these trap (currently only NMI 
and MCE). Do you know when will an NMI happen to a guest, does we support NMI 
watchdog to guest?  How about simply killing the guest if any nest among these 
traps is ok if they are all rare situation. 

> 4) The code in do_iret() doesn't seem to be 64-bit specific at
> all, i.e. I'd
> think this should really be a common subroutine called from all three
> do_iret() handlers (perhaps even including the trap priority
> and affinity
> handling).

As stated above, I have no idea of NMI usage, but for MCE, I suspect if we need 
care about 32bit at all.


> Jan
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