On Wed, Mar 15, 2017 at 03:01:12PM -0400, Gabriel L. Somlo wrote: > On Wed, Mar 15, 2017 at 08:29:23PM +0200, Michael S. Tsirkin wrote: > > 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 > > How would I check for this (I'm sorry, haven't hacked on any KVM > related thing in a while, so I don't have it "cached") :) > > > > > 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)? > > > > > > > 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). > > A few years ago I tried really emulating MONITOR and MWAIT for a > project -- while not a total abject failure, the resulting patch > worked only intermittently (on OS X 10.7, which was the hot new thing > at the time, and hadn't started checking CPUID yet). > > My collected wisdom on the topic from back then is here: > > http://www.contrib.andrew.cmu.edu/~somlo/OSXKVM/mwait.html > > The problem is that MWAIT is required to wake synchronously with > any other "thing" (either another (v)CPU, or DMA, or whatever) writing > to the memory location "marked" by the last preceding MONITOR. While > interrupts of any kind may also wake an MWAIT, it is strictly not allowed > to "miss" a write to the MONITOR-ed memory location. So unless we implement > some sort of condition queue that guarantees re-enabling the "parked" vcpu > on an intercepted write to a specific memory location by another vcpu, > we can't guarantee architecturally correct behavior. > > If linux uses it in a very specific way that can be "faked" even > without ISA compliance, that's OK with me -- but other guest OSs might > take the x86 ISA more literally :) > > Let me know if there's anything else you'd like me to test, now that I > have set up a 4.11.0-rc2+ (a.k.a. kvm git master) testing rig... > > Regards, > --Gabe Doing that corrently in software would be very hard. I suspect your host CPU has an issue, sent a patch to detect that. Let's see what happens. -- MST
