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

["Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>]

Jan Beulich wrote:
>>>> "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx> 26.11.09 10:55 >>>
>> Jan Beulich wrote:
>>> 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?
> 
> No, I'm not talking about both exceptions happening simultaneously,
> but rather about one happening while the other is being serviced (i.e.
> before the corresponding HYPERVISOR_iret was executed).
> 
>> 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.
> 
> Dom0 can get hardware generated NMIs, and any domain can get
> software injected ones.
> 
> And no, killing the VM unless an unhandleable, catastrophic situation
> makes it necessary doesn't seem like a good course of action.

If NMI is used commonly, we of course should not kill them simply because the 
nesting.

> 
>>> 
>>> 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.
> 
> As long as it doesn't require significant effort, I think 32-bit
> guests shouldn't be made second class citizens (and even less if
> they're running on 64-bit Xen). 

Oops, mistaken that I thought it is xen hypervisor :(
Yes, do_iret for 32 bit  guest should be supported also.

--jyh

> 
> Jan
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