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

[Gabriel L. Somlo]

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