On Wed, Mar 15, 2017 at 07:35:34PM -0400, Gabriel L. Somlo wrote: > On Wed, Mar 15, 2017 at 11:22:18PM +0200, Michael S. Tsirkin wrote: > > Guests running Mac OS 5, 6, and 7 (Leopard through Lion) have a problem: > > unless explicitly provided with kernel command line argument > > "idlehalt=0" they'd implicitly assume MONITOR and MWAIT availability, > > without checking CPUID. > > > > We currently emulate that as a NOP but on VMX we can do better: let > > guest stop the CPU until timer, IPI or memory change. CPU will be busy > > but that isn't any worse than a NOP emulation. > > > > Note that mwait within guests is not the same as on real hardware > > because halt causes an exit while mwait doesn't. For this reason it > > might not be a good idea to use the regular MWAIT flag in CPUID to > > signal this capability. Add a flag in the hypervisor leaf instead. > > > > Additionally, we add a capability for QEMU - e.g. if it knows there's an > > isolated CPU dedicated for the VCPU it can set the standard MWAIT flag > > to improve guest behaviour. > > Same behavior (on the mac pro 1,1 running F22 with custom-compiled > kernel from kvm git master, plus this patch on top). > > The OS X 10.7 kernel hangs (or at least progresses extremely slowly) > on boot, does not bring up guest graphical interface within the first > 10 minutes that I waited for it. That, in contrast with the default > nop-based emulation where the guest comes up within 30 seconds. Thanks a lot, meanwhile I'll try to write a unit-test and experiment with various behaviours. > I will run another round of tests on a newer Mac (4-year-old macbook > air) and report back tomorrow. > > Going off on a tangent, why would encouraging otherwise well-behaved > guests (like linux ones, for example) to use MWAIT be desirable to > begin with ? Is it a matter of minimizing the overhead associated with > exiting and re-entering L1 ? Because if so, AFAIR staying inside L1 and > running guest-mode MWAIT in a tight loop will actually waste the host > CPU without the opportunity to yield to some other L0 thread. Sorry if > I fell into the middle of an ongoing conversation on this and missed > most of the relevant context, in which case please feel free to ignore > me... :) > > Thanks, > --G It's just some experiments I'm running, I'm not ready to describe it yet. I thought this part might be useful to at least some guests, so trying to upstream it right now. -- MST
