On 07/12/17 14:11, Jan Beulich wrote:
> As mentioned in Linux commit 87c00572ba ("kvm: x86: emulate monitor and
> mwait instructions as nop"), older OS X versions (for example) may make
> use of the insns without checking CPUID flags (presumably implying
> availability from family/model).

-1 to this.

IIRC, monitor and mwait are disabled entirely due to VMCS/VMCB
configuration, and convert to #UD internally.  The emulator shouldn't be
able to let software work around that.

If and when we decide to support this functionality for guests (which
probably won't be until after EPT SPP gets in), then the feature should
use CPUID as per normal.

There is a large list of other things which prevent OS X from booting
under Xen.  If someone decides to step up and get OS X support working
then we could reconsider whether we quirk this, but until then,
unilaterally quirking it is a net negative.

~Andrew

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