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

[Alexander Graf <agraf@suse.de>]

On 06/03/2014 04:21 PM, 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).

I think we should be able to handle #4 by doing -cpu core2duo,+monitor 
on the QEMU command line which should override the cpuid bits that the 
kernel tells us.


Alex

>
> 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