Re: [PATCH 3/3] KVM: x86: correct mwait and monitor emulation

[Gleb Natapov <gleb@kernel.org>]

On Thu, Jun 19, 2014 at 01:53:36PM +0300, Nadav Amit wrote:
> 
> On Jun 19, 2014, at 1:18 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> 
> > On Wed, Jun 18, 2014 at 02:46:01PM -0400, Gabriel L. Somlo wrote:
> >> On Wed, Jun 18, 2014 at 10:59:14AM -0700, Eric Northup wrote:
> >>> On Wed, Jun 18, 2014 at 7:19 AM, Nadav Amit <namit@cs.technion.ac.il> wrote:
> >>>> mwait and monitor are currently handled as nop. Considering this behavior, they
> >>>> should still be handled correctly, i.e., check execution conditions and generate
> >>>> exceptions when required. mwait and monitor may also be executed in real-mode
> >>>> and are not handled in that case.  This patch performs the emulation of
> >>>> monitor-mwait according to Intel SDM (other than checking whether interrupt can
> >>>> be used as a break event).
> >>>> 
> >>>> Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
> >> 
> >> How about this instead (details in the commit log below) ? Please let
> >> me know what you think, and if you'd prefer me to send it out as a
> >> separate patch rather than a reply to this thread.
> >> 
> >> Thanks,
> >> --Gabriel
> > 
> > If there's an easy workaround, I'm inclined to agree.
> > We can always go back to Gabriel's patch (and then we'll need
> > Nadav's one too) but if we release a kernel with this
> > support it becomes an ABI and we can't go back.
> > 
> > So let's be careful here, and revert the hack for 3.16.
> > 
> > 
> > Acked-by: Michael S. Tsirkin <mst@redhat.com>
> > 
> Personally, I got a custom guest which requires mwait for executing correctly.
Can you elaborate on this guest a little bit. With nop implementation
for mwait the guest will hog a host cpu. Do you consider this to be
"executing correctly?"

--
			Gleb.