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. > 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
