Re: [PATCH v3 07/10] qemu-kvm: Cleanup/fix TSC and PV clock writeback

Marcelo Tosatti
2010-02-25T15:48:52Z

On Thu, Feb 25, 2010 at 04:17:22PM +0100, Jan Kiszka wrote:
> Marcelo Tosatti wrote:
> > On Thu, Feb 25, 2010 at 09:48:47AM +0100, Jan Kiszka wrote:
> >> Marcelo Tosatti wrote:
> >>> On Thu, Feb 25, 2010 at 12:58:26AM +0100, Jan Kiszka wrote:
> >>>> Marcelo Tosatti wrote:
> >>>>> On Thu, Feb 25, 2010 at 12:45:55AM +0100, Jan Kiszka wrote:
> >>>>>> Marcelo Tosatti wrote:
> >>>>>>> On Wed, Feb 24, 2010 at 03:17:55PM +0100, Jan Kiszka wrote:
> >>>>>>>> Drop kvm_load_tsc in favor of level-dependent writeback in
> >>>>>>>> kvm_arch_load_regs. KVM's PV clock MSRs fall in the same category and
> >>>>>>>> should therefore only be written back on full sync.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> >>>>>>>> ---
> >>>>>>>> qemu-kvm-x86.c | 19 +++++--------------
> >>>>>>>> qemu-kvm.h | 4 ----
> >>>>>>>> target-i386/machine.c | 5 -----
> >>>>>>>> 3 files changed, 5 insertions(+), 23 deletions(-)
> >>>>>>>>
> >>>>>>>> diff --git a/qemu-kvm-x86.c b/qemu-kvm-x86.c
> >>>>>>>> index 840c1c9..84fd7fa 100644
> >>>>>>>> --- a/qemu-kvm-x86.c
> >>>>>>>> +++ b/qemu-kvm-x86.c
> >>>>>>>> @@ -965,8 +965,11 @@ void kvm_arch_load_regs(CPUState *env, int level)
> >>>>>>>> set_msr_entry(&msrs[n++], MSR_LSTAR , env->lstar);
> >>>>>>>> }
> >>>>>>>> #endif
> >>>>>>>> - set_msr_entry(&msrs[n++], MSR_KVM_SYSTEM_TIME, env->system_time_msr);
> >>>>>>>> - set_msr_entry(&msrs[n++], MSR_KVM_WALL_CLOCK, env->wall_clock_msr);
> >>>>>>>> + if (level == KVM_PUT_FULL_STATE) {
> >>>>>>>> + set_msr_entry(&msrs[n++], MSR_IA32_TSC, env->tsc);
> >>>>>>>> + set_msr_entry(&msrs[n++], MSR_KVM_SYSTEM_TIME, env->system_time_msr);
> >>>>>>>> + set_msr_entry(&msrs[n++], MSR_KVM_WALL_CLOCK, env->wall_clock_msr);
> >>>>>>>> + }
> >>>>>>> As things stand today, the TSC should only be written on migration. See
> >>>>>>> 53f658b3c33616a4997ee254311b335e59063289 in the kernel.
> >>>>>> Migration and power-up - that's what this patch ensures (=>
> >>>>>> KVM_PUT_FULL_STATE). Or where do you see any problem?
> >>>>>>
> >>>>>> Jan
> >>>>>>
> >>>>> The problem is it should not write on power up (the kernel attempts
> >>>>> to synchronize the TSCs in that case, see the commit).
> >>>>>
> >>>> OK, need to read this more carefully.
> >>>>
> >>>> I do not yet understand the difference from user space POV: it tries to
> >>>> transfer the identical TSC values to all currently stopped VCPU threads.
> >>> guest tsc = host tsc + offset
> >>>
> >>> So at the time you set_msr(TSC), the guest visible TSC starts ticking.
> >>> For SMP guests, this does not happen exactly at the same time for all
> >>> vcpus.
> >> Ouch.
> >>
> >>>> That should not be different if we are booting a fresh VM or loading a
> >>>> complete state of a migrated image. If it does, it looks like a KVM
> >>>> kernel deficit on first glance.
> >>> Yes it is a deficit. After migration TSCs of SMP guests go out of sync.
> >>> Zachary is working on that.
> >>>
> >> OK, so we need a workaround, ideally without reintroducing hooks. Is
> >> this one acceptable?
> >>
> >>
> >> diff --git a/qemu-kvm-x86.c b/qemu-kvm-x86.c
> >> index 84fd7fa..285c05a 100644
> >> --- a/qemu-kvm-x86.c
> >> +++ b/qemu-kvm-x86.c
> >> @@ -966,7 +966,15 @@ void kvm_arch_load_regs(CPUState *env, int level)
> >> }
> >> #endif
> >> if (level == KVM_PUT_FULL_STATE) {
> >> - set_msr_entry(&msrs[n++], MSR_IA32_TSC, env->tsc);
> >> + /*
> >> + * KVM is yet unable to synchronize TSC values of multiple VCPUs on
> >> + * writeback. Until this is fixed, we only write the offset to SMP
> >> + * guests after migration, desynchronizing the VCPUs, but avoiding
> >> + * huge jump-backs that would occur without any writeback at all.
> >> + */
> >> + if (smp_cpus == 1 || env->tsc != 0) {
> >> + set_msr_entry(&msrs[n++], MSR_IA32_TSC, env->tsc);
> >> + }
> >> set_msr_entry(&msrs[n++], MSR_KVM_SYSTEM_TIME, env->system_time_msr);
> >> set_msr_entry(&msrs[n++], MSR_KVM_WALL_CLOCK, env->wall_clock_msr);
> >> }
> >
> > Well, migration of the SMP TSCs is not precise, but it is better than
> > zeroing the TSCs on migration. So something like a new state (migration
> > only), or a hack that mimicks that behaviour, is needed :(
>
> It is not precise, but the above code shouldn't behave differently
> compared to the existing one: tcp != 0 => we are loading values from
> some VM that ran before, i.e. we are migrating.
>
> If somehow possible, I do not want to introduce a new writeback level
> for an x86-only kernel quirk.
>
> Jan

I can't read. Looks OK.