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Re: [Qemu-devel] [PATCH v6] kvm: better MWAIT emulation for guests

From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH v6] kvm: better MWAIT emulation for guests
Date: Wed, 12 Apr 2017 16:54:10 +0200
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

On 12.04.17 16:34, Jim Mattson wrote:
Actually, we have rejected commit 87c00572ba05aa8c ("kvm: x86: emulate
monitor and mwait instructions as nop"), so when we intercept
MONITOR/MWAIT, we synthesize #UD. Perhaps it is this difference from
vanilla kvm that motivates the following idea...

So you're not running upstream kvm? In that case, you can just not take this patch either :).

Since we're still not going to report MONITOR support in CPUID, the
only guests of consequence are paravirtual guests. What if a

Only if someone actually implemented something for PV guests, yes.

The real motivation is to allow user space to force set the MONITOR CPUID flag. That way an admin can - if he really wants to - dedicate pCPUs to the VM.

I agree that we don't need the kvm pv flag for that. I'd be happy to drop that if everyone agrees.

paravirtual guest was aware of the fact that sometimes MONITOR/MWAIT
would work as architected, and sometimes they would raise #UD (or do
something else that's guest-visible, to indicate that the hypevisor is
intercepting the instructions). Such a guest could first try a
MONITOR/MWAIT-based idle loop and then fall back on a HLT-based idle
loop if the hypervisor rejected its use of MONITOR/MWAIT.

How would that work? That guest would have to atomically notify all other vCPUs that wakeup notifications now go via IPIs instead of cache line dirtying.

That's probably as much work to get right as it would be to just emulate MWAIT inside kvm ;).

We already have the loose concept of "this pCPU has other things to
do," which is encoded in the variable-sized PLE window. With
MONITOR/MWAIT, the choice is binary, but a simple implementation could
tie the two together, by allowing the guest to use MONITOR/MWAIT
whenever the PLE window exceeds a certain threshold. Or the decision
could be left to the userspace agent.

I agree, and that's basically the idea I mentioned earlier with MWAIT emulation. We could (for well behaved guests) switch between emulating MWAIT and running native MWAIT.


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