Re: [Xen-devel] [PATCH v4] x86/AMD: Add support for AMD's OSVW feature in guests

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.02.12 at 17:13, Boris Ostrovsky <boris.ostrovsky@xxxxxxx> wrote:
> On 02/03/12 02:53, Jan Beulich wrote:
>>>>> On 02.02.12 at 21:29, Boris Ostrovsky<boris.ostrovsky@xxxxxxx>  wrote:
>>> On 02/02/12 08:22, Jan Beulich wrote:
>>>> As I was about to apply this to my local tree to give it a try, I had
>>>> to realize that the microcode integration is still not correct: There
>>>> is (at least from an abstract perspective) no guarantee for
>>>> cpu_request_microcode() to be called on all CPUs, yet you want
>>>> svm_host_osvw_init() to be re-run on all of them. If you choose
>>>> to not deal with this in a formally correct way, it should be stated
>>>> so in a code comment (to lower the risk of surprises when someone
>>>> touches that code) that this is not possible in practice because
>>>> collect_cpu_info() won't currently fail for CPUs of interest.
>>>
>>> What if svm_host_osvw_init() is called from collect_cpu_info()? There
>>> may be cases when svm_host_osvw_init() is called multiple times for the
>>> same cpu but that should be harmless (and the routine will be renamed to
>>> svm_host_osvw_update()).
>>
>> Wouldn't that result in workaround bits that might get cleared with
>> the pending microcode update to get (and remain) set, as they're
>> being or-ed together over all invocations of the function after any
>> svm_host_osvw_reset()?
> 
> 
> I think that would be an OK but not optimal situation: more bits will 
> end up being set than necessary, meaning that workarounds will need to 
> be applied where they may not be required. But that should not affect 
> correctness. I am not sure it's worth optimizing for this case since I 
> think onlining a core while doing a microcode update is a rather 
> uncommon occurrence.

How is that related to CPU onlining? If you put the call into
collect_cpu_info(), that'll affect all CPUs currently online as well
(and, as I said, negate the effect of possibly cleared OSVW bits
that the pending microcode updated might have). The only case
where calling svm_host_osvw_init() from collect_cpu_info() would
be correct is if the latter returns an error. Which, as I noted
before, can only happen for non-AMD or pre-Fam10 CPUs (on
all of which calling the function is pointless). So we're back to me
asking that this simply be explained in a code comment in lieu of
a proper abstraction of the situation.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel