Re: [PATCH] kvm: better MWAIT emulation for guests

["Michael S. Tsirkin" <mst@redhat.com>]

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