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Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests

From Alexander Graf <agraf@suse.de>
Newsgroups linux.kernel
Subject Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests
Date 2017-03-27 15:40 +0200
Message-ID <tpD2h-kn-21@gated-at.bofh.it> (permalink)
References <tloEy-50F-19@gated-at.bofh.it>
Organization linux.* mail to news gateway

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On 15/03/2017 22:22, Michael S. Tsirkin wrote:
> Guests running Mac OS 5, 6, and 7 (Leopard through Lion) have a problem:
> unless explicitly provided with kernel command line argument
> "idlehalt=0" they'd implicitly assume MONITOR and MWAIT availability,
> without checking CPUID.
>
> We currently emulate that as a NOP but on VMX we can do better: let
> guest stop the CPU until timer, IPI or memory change.  CPU will be busy
> but that isn't any worse than a NOP emulation.
>
> Note that mwait within guests is not the same as on real hardware
> because halt causes an exit while mwait doesn't.  For this reason it
> might not be a good idea to use the regular MWAIT flag in CPUID to
> signal this capability.  Add a flag in the hypervisor leaf instead.

So imagine we had proper MWAIT emulation capabilities based on page 
faults. In that case, we could do something as fancy as

Treat MWAIT as pass-through by default

Have a per-vcpu monitor timer 10 times a second in the background that 
checks which instruction we're in

If we're in mwait for the last - say - 1 second, switch to emulated 
MWAIT, if $IP was in non-mwait within that time, reset counter.

Or instead maybe just reuse the adapter hlt logic?

Either way, with that we should be able to get super low latency IPIs 
running while still maintaining some sanity on systems which don't have 
dedicated CPUs for workloads.

And we wouldn't need guest modifications, which is a great plus. So 
older guests (and Windows?) could benefit from mwait as well.


Alex

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Thread

[PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-15 22:30 +0100
  Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 00:40 +0100
    Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-16 00:50 +0100
      Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 14:30 +0100
        Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-03-16 15:10 +0100
          Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 17:00 +0100
            Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-03-16 17:00 +0100
              Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 17:30 +0100
        Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-16 15:10 +0100
          Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 16:00 +0100
            Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-16 16:30 +0100
            Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-03-16 16:40 +0100
              Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-03-16 17:10 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 17:50 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-16 18:30 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 18:50 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-16 19:40 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-16 20:30 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 21:20 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 22:20 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-17 03:10 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-17 14:40 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-21 04:30 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-03-21 18:10 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Nadav Amit <nadav.amit@gmail.com> - 2017-03-21 18:40 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-03-21 20:30 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Gabriel Somlo <gsomlo@gmail.com> - 2017-03-22 00:00 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Nadav Amit <nadav.amit@gmail.com> - 2017-03-22 01:10 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-22 14:40 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-22 15:20 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-22 15:20 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 20:30 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-03-16 18:30 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 18:40 +0100
              Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 17:20 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-16 17:50 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 18:00 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-16 18:00 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-16 18:20 +0100
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-03-16 18:40 +0100
  Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Joerg Roedel <joro@8bytes.org> - 2017-03-21 17:20 +0100
    Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-21 19:50 +0100
  Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Alexander Graf <agraf@suse.de> - 2017-03-27 15:40 +0200
    Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-03-28 16:40 +0200
      Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Jim Mattson <jmattson@google.com> - 2017-03-28 22:40 +0200
        Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-03-29 14:20 +0200
          Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Alexander Graf <agraf@suse.de> - 2017-04-03 12:10 +0200
            Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-04-04 14:40 +0200
              Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Alexander Graf <agraf@suse.de> - 2017-04-04 15:00 +0200
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Alexander Graf <agraf@suse.de> - 2017-04-04 15:20 +0200
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-04-04 15:50 +0200
                Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-04-04 15:20 +0200

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