Re: [Xen-devel] [PATCH] x86/cpuid: Avoid unconditionally clobbering ITSC for guests

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.05.16 at 18:59, <andrew.cooper3@xxxxxxxxxx> wrote:
> In general, Invariant TSC is not a feature which can be advertised to guests,
> because it cannot be guaranteed across migrate.  domain_cpuid() goes so far as
> to deliberately clobber the feature flag under a number of circumstances.
> 
> Because ITSC is absent from the static {pv,hvm}_featureset masks, c/s b648feff
> "xen/x86: Improvements to in-hypervisor cpuid sanity checks" caused ITSC to be
> unconditionally masked out.
> 
> As an interim solution, include the hosts idea of ITSC along with the static
> {pv,hvm}_featureset when restricting the guests view of features.  This causes
> the hardware domain, and VMs explicitly configured with ITSC and no-migrate to
> be offered ITSC (subject to hardware availability).
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>


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