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

From: Jim Mattson
Subject: Re: [Qemu-devel] [PATCH v6] kvm: better MWAIT emulation for guests
Date: Wed, 12 Apr 2017 08:20:21 -0700

On Wed, Apr 12, 2017 at 7:54 AM, Alexander Graf <address@hidden> wrote:
> 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 :).
This patch should be harmless. :-)
>> 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 ;).
True. I don't have an easy solution to that problem.
>> 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.
Yes, that would probably be the preferred solution.
> Alex

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