kvmclock doesn't work, help?

Andy Lutomirski
2015-12-09T21:11:20Z

I'm trying to clean up kvmclock and I can't get it to work at all. My
host is 4.4.0-rc3-ish on a Skylake laptop that has a working TSC.

If I boot an SMP (2 vcpus) guest, tracing says:

qemu-system-x86-2517 [001] 102242.610654: kvm_update_master_clock:
masterclock 0 hostclock tsc offsetmatched 0
qemu-system-x86-2521 [000] 102242.613742: kvm_track_tsc:
vcpu_id 0 masterclock 0 offsetmatched 0 nr_online 1 hostclock tsc
qemu-system-x86-2522 [000] 102242.622959: kvm_track_tsc:
vcpu_id 1 masterclock 0 offsetmatched 1 nr_online 2 hostclock tsc
qemu-system-x86-2521 [000] 102242.645123: kvm_track_tsc:
vcpu_id 0 masterclock 0 offsetmatched 1 nr_online 2 hostclock tsc
qemu-system-x86-2522 [000] 102242.647291: kvm_track_tsc:
vcpu_id 1 masterclock 0 offsetmatched 1 nr_online 2 hostclock tsc
qemu-system-x86-2521 [000] 102242.653369: kvm_track_tsc:
vcpu_id 0 masterclock 0 offsetmatched 1 nr_online 2 hostclock tsc
qemu-system-x86-2522 [000] 102242.653429: kvm_track_tsc:
vcpu_id 1 masterclock 0 offsetmatched 1 nr_online 2 hostclock tsc
qemu-system-x86-2517 [001] 102242.653447: kvm_update_master_clock:
masterclock 0 hostclock tsc offsetmatched 1
qemu-system-x86-2521 [000] 102242.653657: kvm_update_master_clock:
masterclock 0 hostclock tsc offsetmatched 1
qemu-system-x86-2522 [002] 102242.664448: kvm_update_master_clock:
masterclock 0 hostclock tsc offsetmatched 1

If I boot a UP guest, tracing says:

qemu-system-x86-2567 [001] 102370.447484: kvm_update_master_clock:
masterclock 0 hostclock tsc offsetmatched 1
qemu-system-x86-2571 [002] 102370.447688: kvm_update_master_clock:
masterclock 0 hostclock tsc offsetmatched 1

I suspect, but I haven't verified, that this is fallout from:

commit 16a9602158861687c78b6de6dc6a79e6e8a9136f
Author: Marcelo Tosatti <mtosatti@redhat.com>
Date: Wed May 14 12:43:24 2014 -0300

KVM: x86: disable master clock if TSC is reset during suspend

Updating system_time from the kernel clock once master clock
has been enabled can result in time backwards event, in case
kernel clock frequency is lower than TSC frequency.

Disable master clock in case it is necessary to update it
from the resume path.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Can we please stop making kvmclock more complex? It's a beast right
now, and not in a good way. It's far too tangled with the vclock
machinery on both the host and guest sides, the pvclock stuff is not
well thought out (even in principle in an ABI sense), and it's never
been clear to my what problem exactly the kvmclock stuff is supposed
to solve.

I'm somewhat tempted to suggest that we delete kvmclock entirely and
start over. A correctly functioning KVM guest using TSC (i.e.
ignoring kvmclock entirely) seems to work rather more reliably and
considerably faster than a kvmclock guest.

--Andy

--
Andy Lutomirski
AMA Capital Management, LLC