On Wed, Mar 15, 2017 at 02:14:26PM -0400, Gabriel L. Somlo wrote: > Michael, > > I tested this on OS X 10.7 (Lion), the last version that doesn't check > CPUID for MWAIT support. > > I used the latest kvm from git://git.kernel.org/pub/scm/virt/kvm/kvm.git > first as-is, then with your v2 MWAIT patch applied. > > Single-(V)CPU guest works as expected (but then again, single-vcpu > guests worked even back when I tried emulating MWAIT the same as HLT). > > When I try starting a SMP guest (with "-smp 4,cores=2"), the guest OS > hangs after generating some output in text/verbose boot mode -- I gave > up waiting for it after about 5 minutes. Works fine before your patch, > which leads me to suspect that, as I feared, MWAIT doesn't wake > immediately upon another VCPU writing to the MONITOR-ed memory location. > > Tangentially, I remember back in the days of OS X 10.7, the > alternative to exiting guest mode and emulating MWAIT and MONITOR as > NOPs was to allow them both to run in guest mode. > > While poorly documented by Intel at the time, MWAIT at L>0 effectively > behaves as a NOP (i.e., doesn't actually put the physical core into > low-power mode, because doing that would allow a guest to effectively > DOS the host hardware). Thanks for the testing, interesting. Testing with Linux guest seems to show it works. This could be an interrupt thing not a monitor thing. Question: does your host CPU have this in its MWAIT leaf? Bit 01: Supports treating interrupts as break-event for MWAIT, even when interrupts disabled We really should check that before enabling, I'll add that. > > Given how unusual it is for a guest to use MONITOR/MWAIT in the first > place, what's wrong with leaving it all as is (i.e., emulated as NOP)? > > Thanks, > --Gabriel I'm really looking into ways to use mwait within Linux guests, this is just a building block that should help Mac OSX as a side effect (and we do not want it broken if at all possible). -- MST
