Re: [PATCH v2] kvm: x86: emulate monitor and mwait instructions as nop

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 03, 2014 at 10:21:58AM -0400, Gabriel L. Somlo wrote:
> On Tue, Jun 03, 2014 at 11:17:48AM +0200, Paolo Bonzini wrote:
> > 
> > I think it's fine as it is now. :)
> 
> On Mon, Jun 02, 2014 at 09:55:18PM -0400, Gabriel L. Somlo wrote:
> > 
> > W.r.t. monitor/mwait, a guest can do one of the following:
> > 
> > 1. Never check CPUID, and never use monitor/mwait
> > 	- This is great, we don't have to do anything about these
> > 
> > 2. Check CPUID for mwait, use it to idle in preference over hlt
> > 	- Linux, Windows, and Mavericks (10.9) do this
> > 	- we never want to have CPUID say "yes" to these, since
> > 	  monitor/mwait support will be clunky in the best case,
> > 	  and hlt is overwhelmingly preferable! [*]
> > 
> > 3. Never check CPUID, use monitor/mwait with abandon
> > 	- OS X 10.6 .. 10.8 does this
> > 	- emulating monitor/mwait here allows us to boot the guest
> > 	  and use it, and perform sysadmin surgery to force a hlt
> > 	  based idle
> > 
> > 4. Check CPUID, panic if unavailable
> > 	- OS X 10.5 did this, IIRC.
> > 	- whether I can do kext surgery and get it to stop checking
> > 	  CPUID *in addition to* falling back to hlt-based idle is
> > 	  TBD.
> > 	- emulating monitor/mwait allows us to boot this type of
> > 	  guest, BUT WE ALSO HAVE TO ADVERTISE IT VIA CPUID !!!
> 
> As it is right now, #4 is not being addressed (and we can't just
> advertise mwait via cpuid, or we'd be screwing up #2).

Yes, I didn't understand 10.5 did #4.

> I also feel a bit weird about the "undocumented feature" aspect
> of NOT generating an invalid opcode for something that *should*
> be an invalid opcode according to the feature set advertised via
> cpuid...
> 
> So if there's a way to make it so we can tell QEMU/KVM to
> "--enable-mwait" on a per-guest basis, I think that'd be better
> than an always-on "undocumented" behavior...
> 
> But then again, I'm most likely missing something about the big
> picture... :)
> 
> Thanks much,
> --Gabriel