Re: [Xen-devel] what will happen for a floated vcpu?

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Thu, 2016-03-31 at 16:27 +0000, tutu sky wrote:
> Thanks Dario,
> Yeah, you are totally right, but software crashes are almost a result
> of hardware faults. 
>
Well, no, I'd sa that software crashes are almost always due to bugs in
the software itself. Then there are also hardware faults.

> I mean if a part of a core for example register file or maybe
> floating point unit or even ALU, suddenly stop working or does
> malfunction, what will happen for the corresponding vcpu which it is
> running? Does that vcpu crash (as a part of a software) or migrate to
> another core? 
>
I still don't follow. When you say "register file or ALU stop working
or malfunction", that does not mean much, unless better specify what
malfunction means.

For instance, if a CPU starts to behave in such a way that you store
0xabcd in register R1 and then, when you later read it, you read
0xdcba, what would happen? How does Xen (or any other hypervisor or OS)
knows this is a malfunctioning. Well, that entirely depend of what
specific piece of code is running when the malfunctioning manifests,
and what will likely happen is that the execution of the software would
be altered in a way that the software was not expecting and, at some
point, things will crash. Whether it would be the hypervisor to crash,
or the OS of a guest, or a user level process in a guest, is again
dependant on a lot of things.

So, really, to a question like "what happens to a Xen system if ALU
starts to malfunction?", the only possible answer is "bad things, but
who knows".

> You just please imagine recoverable faults. Has Xen any plan for deal
> with such a situation? 
>
The "situation" you're asking about is too broad a thing to have a plan
for it, as a whole.

For specific issues, there may or may not be plans or actually
implemented solution.

For examples (but I don't really know if this could be related to what
you're thinking about) there is this:
https://en.wikipedia.org/wiki/Machine-check_exception

And Xen does some MCE stuff (but I don't know the details myself).

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
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Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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