Re: [Xen-devel] [PATCH 04/20] xen/domctl: Drop vcpu_alloc_lock

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.03.18 at 20:13, <andrew.cooper3@xxxxxxxxxx> wrote:
> It is not entirely clear why this interlock was introduced in c/s 8cbb5278e
> "x86/AMD: Add support for AMD's OSVW feature in guests".
> 
> At the time, svm_handle_osvw() could have seen an unexpected change in OSVW
> (not the case now, due to the new CPUID Policy infrastructure), but even then,
> it would have caused spurious changes in behaviour when handling
> OSVW_{ID_LENGTH,STATUS} read requests on behalf of an already-running guest.
> 
> There are plenty of other aspects of domain creation which depend on hardware
> details which may change across a microcode load, but where not protected by
> this interlock.

Are there? We don't re-read CPUID (yet), for example. But of
course it is also not really specified which aspects may change
across microcode updates.

> A host administrator choosing to perform late microcode loading has plenty of
> other problems to worry about, and is it not unreasonable to expect them to
> temporarily cease domain construction activities while the microcode loading
> is in progress.

But it is also not unreasonable to expect the hypervisor to guard
against inconsistencies here. On the whole I'm not really
convinced; I think I'd like to hear others' opinions.

Jan


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