[Xen-devel] RE: expose MWAIT to dom0

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 15.08.11 at 09:57, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> So how about the change like below?
> 
> emulate_forced_invalid_op:
> -        __clear_bit(X86_FEATURE_MWAIT % 32, &c);
> +        if ( !IS_PRIV(current->domain) || !xen_cpuidle )
> +            __clear_bit(X86_FEATURE_MWAIT % 32, &c);

You'd break any Dom0 kernel that uses the flag to determine whether it
can actually use the mwait/monitor instructions. I agree with Keir that
for this particular purpose the code ought to look at the real CPUID
flag.

It should be possible to do this without changing non-Xen code, since
the use of the instructions is additionally gated on CPUID leaf 5
producing non-zero output. But again, the kernel has to do this for
itself, the hypervisor shouldn't expose the feature flag.

Jan


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