On Wed, Mar 15, 2017 at 03:43:03PM -0400, Gabriel L. Somlo wrote: > Applies cleanly over git://git.kernel.org/pub/scm/virt/kvm/kvm.git, > but then I get: > > CC [M] arch/x86/kvm/x86.o > In file included from arch/x86/kvm/x86.c:28:0: > arch/x86/kvm/x86.h: In function ‘kvm_mwait_in_guest’: > arch/x86/kvm/x86.h:231:34: error: ‘CPUID_MWAIT_LEAF’ undeclared (first use in this function) > if (boot_cpu_data.cpuid_level < CPUID_MWAIT_LEAF) > ^ > arch/x86/kvm/x86.h:231:34: note: each undeclared identifier is reported only once for each function it appears in > arch/x86/kvm/x86.h:234:45: error: ‘mwait_substates’ undeclared (first use in this function) > cpuid(CPUID_MWAIT_LEAF, &eax, &ebx, &ecx, &mwait_substates); > ^ > arch/x86/kvm/x86.h:236:14: error: ‘CPUID5_ECX_INTERRUPT_BREAK’ undeclared (first use in this function) > if (!(ecx & CPUID5_ECX_INTERRUPT_BREAK)) > ^ > arch/x86/kvm/x86.h:238:1: warning: control reaches end of non-void function [-Wreturn-type] > } > ^ > scripts/Makefile.build:294: recipe for target 'arch/x86/kvm/x86.o' failed > make[2]: *** [arch/x86/kvm/x86.o] Error 1 > scripts/Makefile.build:553: recipe for target 'arch/x86/kvm' failed > make[1]: *** [arch/x86/kvm] Error 2 > Makefile:1002: recipe for target 'arch/x86' failed > make: *** [arch/x86] Error 2 forgot to commit :( Will resend, sorry. > > Did you accidentally leave out something that went into a .h file > somewhere ? > > Thx, > --G > > On Wed, Mar 15, 2017 at 09:29:57PM +0200, Michael S. Tsirkin wrote: > > 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
