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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxx>]

On 02/03/12 11:25, Jan Beulich wrote:
> > > > 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.

OK, I misunderstood your original comment then. I thought you were 
talking about a race between microcode update and onlining a core.
-boris


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