Re: [Xen-devel] [PATCH] xen/domctl: Drop vcpu_alloc_lock

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.09.18 at 11:57, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 07/09/18 09:48, Jan Beulich wrote:
>> Rather than blindly dropping the logic, I'd have expected
>> for it to be fixed: Despite the movement into XEN_DOMCTL_createdomain
>> there's still a race between ucode updates and domain creation.
> 
> What race?  What have I overlooked?

One CPU doing a microcode update while the other creates a
domain. I haven't looked in detail, but I think all domain creation
should be deferred until completion of the microcode update, so
that domains get a consistent/predictable OSVW state set up.

Of course we can put ourselves on the position that it is an
admin mistake to invoke a microcode update without suspending
domain creations temporarily, but since domain creation includes
domain reboots, I don't think this is something an admin can
fully control.

Yet further we could put ourselves on the position that ucode
updates with _any_ guests running are a bad idea.

Jan



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