On Thu, Jun 05, 2014 at 01:59:17PM -0700, Eric Northup wrote: > On Wed, May 7, 2014 at 1:52 PM, Gabriel L. Somlo <gsomlo@xxxxxxxxx> wrote: > > Treat monitor and mwait instructions as nop, which is architecturally > > correct (but inefficient) behavior. We do this to prevent misbehaving > > guests (e.g. OS X <= 10.7) from crashing after they fail to check for > > monitor/mwait availability via cpuid. > > > > Since mwait-based idle loops relying on these nop-emulated instructions > > would keep the host CPU pegged at 100%, do NOT advertise their presence > > via cpuid, to prevent compliant guests from using them inadvertently. > > If it's going to peg the host CPU at 100% anyway, why bother emulating > it? Just let the guest run the mwait instruction! Have a condition > that controls whether CPU_BASED_MWAIT_EXITING gets set in the VMCS > processor execution controls. Go ahead and put it in CPUID, since > it's actually allowed. If a well-behaved guest *does* check CPUID, we want it to get a negative answer and fall back to a HLT-based idle loop (rather than use mwait and peg the host cpu to 100%). We only emulate it so that guests which use it blindly, regardless of what CPUID says (e.g., OS X 10.6 .. 10.8) would still run instead of crashing with an invalid opcode fault. That is, work long enough for someone to manually force a hlt-idle fallback on the guest-side by removing System/Library/Extensions/AppleIntelCPUPowerManagement.kext :) Causing a VM exit (as opposed to letting the MWAIT just be a NOP in VM guest mode) is nice, since it allows us to try and do something more fancy in the future (i.e. yield the host cpu instead of re-entering immediately, or attempting to otherwise emulate the instruction in a way that's "better"). Anyhow, my $0.02 :) Cheers, --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
