Re: TSC frequency configuration & invtsc migration (was Re: [PATCH 4/4] kvm: Allow migration with invtsc)

Marcelo Tosatti
2017-01-04T22:26:48Z

On Wed, Jan 04, 2017 at 05:59:17PM -0200, Eduardo Habkost wrote:
> On Wed, Jan 04, 2017 at 11:39:16AM -0200, Eduardo Habkost wrote:
> > On Wed, Jan 04, 2017 at 09:56:56AM -0200, Marcelo Tosatti wrote:
> > > On Tue, Dec 27, 2016 at 05:21:20PM -0200, Eduardo Habkost wrote:
> > > > Instead of blocking migration on the source when invtsc is
> > > > enabled, rely on the migration destination to ensure there's no
> > > > TSC frequency mismatch.
> > > >
> > > > We can't allow migration unconditionally because we don't know if
> > > > the destination is a QEMU version that is really going to ensure
> > > > there's no TSC frequency mismatch. To ensure we are migrating to
> > > > a destination that won't ignore SET_TSC_KHZ errors, allow invtsc
> > > > migration only on pc-*-2.9 and newer.
> > > >
> > > > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> [...]
> > > > @@ -2655,12 +2656,14 @@ int kvm_arch_put_registers(CPUState *cpu, int level)
> > > > }
> > > >
> > > > if (level == KVM_PUT_FULL_STATE) {
> > > > - /* We don't check for kvm_arch_set_tsc_khz() errors here,
> > > > - * because TSC frequency mismatch shouldn't abort migration,
> > > > - * unless the user explicitly asked for a more strict TSC
> > > > - * setting (e.g. using an explicit "tsc-freq" option).
> > > > + /* Migration TSC frequency mismatch is fatal only if we are
> > > > + * actually reporting Invariant TSC to the guest.
> > > > */
> > > > - kvm_arch_set_tsc_khz(cpu);
> > > > + ret = kvm_arch_set_tsc_khz(cpu);
> > > > + if ((x86_cpu->env.features[FEAT_8000_0007_EDX] & CPUID_APM_INVTSC) &&
> > > > + ret < 0) {
> > > > + return ret;
> > > > + }
> > > > }
> > >
> > > Will the guest continue in the source in this case?
> > >
> > > I think this is past the point where migration has been declared
> > > successful.
> > >
> > > Otherwise looks good.
> >
> > Good point. I will make additional tests and see if there's some
> > other place where the kvm_arch_set_tsc_khz() call can be moved
> > to.
>
> So, if we solve this and do something on (for example) post_load,
> we still have a problem: device state is migrated after RAM. This
> means QEMU will check for TSC scaling and abort migration very
> late.
>
> We could solve that by manually registering a SaveVMHandler that
> will send the TSC frequency on save_live_setup, so migration is
> aborted earlier.
>
> But: this sounds like just a complex hack to work around the real
> problems:
>
> 1) TSC frequency is guest-visible, and anything that affects
> guest ABI should depend on the VM configuration, not on host
> capabilities;

Well not really: the TSC frequency where the guest starts
is the frequency the guest software expects.
So it does depend on host capabilities.

> 2) Setting TSC frequency depending on the host will make
> migratability unpredictable for management software: the same
> VM config could be migratable to host A when started on host
> B, but not migratable to host A when started on host C.

Well, just check the frequency.

> I suggest we allow migration with invtsc if and only if
> tsc-frequency is set explicitly by management software. In other
> words, apply only patches 1/4 and 2/4 from this series. After
> that, we will need libvirt support for configuring tsc-frequency.

I don't like that for the following reasons:

* It moves low level complexity from QEMU/KVM to libvirt
(libvirt has to call KVM_GET_TSC_KHZ from a vcpu thread).
* It makes it difficult to run QEMU manually (i use QEMU
manually all the time).
* It requires patching libvirt.

In my experience things work better when the functionality is
not split across libvirt/qemu.

Can't this be fixed in QEMU? Just check that destination host supports
TSC scaling before migration (or that KVM_GET_TSC_KHZ return value
matches on source and destination).