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

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

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

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)?
> 
> Thanks,
> --Gabriel

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).

-- 
MST