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

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

On Wed, Jun 04, 2014 at 03:24:06PM -0400, Gabriel L. Somlo wrote:
> On Wed, Jun 04, 2014 at 10:06:18PM +0300, Michael S. Tsirkin wrote:
> > On Wed, Jun 04, 2014 at 01:07:21PM -0400, Gabriel L. Somlo wrote:
> > > Ah, so kvm_vcpu_ioctl_set_cpuid() and friends, morally similar to
> > > kvm_vcpu_ioctl_enable_cap() on ppc, except it turns on cpuid flags
> > > instead of entire kvm capabilities.
> > > 
> > > So we either have
> > > 
> > > 	1 always-on but masked-by-default monitor/mwait as
> > > 	  nop, and enable just the cpuid flag on demand via the
> > > 	  existing ioctl_enable_cap() mechanism (and I have to
> > > 	  check out the qemu parser for cpuid command-line flags),
> > > 
> > > or
> > > 
> > > 	2 off-by-default monitor/mwait/cpuid-flag, enabled via
> > > 	  ioctl_enable_cap(), which would have to first be ported
> > > 	  to x86, and would require somewhat more extensive qemu
> > > 	  hackery to take advantage of.
> > > 
> > > I think I sense a "path of least resistance" here, even though IMHO
> > > #2 is still "The Right Thing To Do (TM)"  :) :)
> > > 
> > > Thanks,
> > > --Gabriel
> > 
> > I think it's worng.
> > We really can't emulate mwait at the moment.
> > All we manage to do is a work-around for broken guests.
> > 
> > So let's not pretend that we can, just enable nop
> > unconditionally and be done with it.
> > Paolo already said it's OK with him, and I'll ack too.
> > 
> > Otherwise you are giving bad information to well-behaved guests,
> > so e.g. linux will try to use mwait. You don't want this.
> 
> That's why I suggested having it default to disabled, and only
> allowing it to be turned on per VM, explicitly.

Turn on the bit in CPUID? Why is it useful?
I thought the point of the nop hack is work around
for broken guests.

> > The advantage is that if at some point CPUs can
> > actually support mwait in VMs, at that point
> > we will enable the CPUID bit, and userspace and guests
> > will be able to detect that and rely on that bit
> > to mean "mwait works and is efficient".
> 
> OK, that makes sense. Thanks for helping me finally get over it ;)
> 
> --Gabriel