Re: [Qemu-devel] [PATCH v2 0/3] target-i386: save/restore vcpu's TSC rate during migration

[Haozhong Zhang]

On Fri, Oct 23, 2015 at 08:35:20AM -0200, Marcelo Tosatti wrote:
> On Thu, Oct 22, 2015 at 04:45:21PM -0200, Eduardo Habkost wrote:
> > On Tue, Oct 20, 2015 at 03:22:51PM +0800, Haozhong Zhang wrote:
> > > This patchset enables QEMU to save/restore vcpu's TSC rate during the
> > > migration. When cooperating with KVM which supports TSC scaling, guest
> > > programs can observe a consistent guest TSC rate even though they are
> > > migrated among machines with different host TSC rates.
> > > 
> > > A pair of cpu options 'save-tsc-freq' and 'load-tsc-freq' are added to
> > > control the migration of vcpu's TSC rate.
> > 
> > The requirements and goals aren't clear to me. I see two possible use
> > cases, here:
> > 
> > 1) Best effort to keep TSC frequency constant if possible (but not
> >    aborting migration if not possible). This would be an interesting
> >    default, but a bit unpredictable.
> > 2) Strictly ensuring TSC frequency stays constant on migration (and
> >    aborting migration if not possible). This would be an useful feature,
> >    but can't be enabled by default unless both hosts have the same TSC
> >    frequency or support TSC scaling.
> 
> Only destination needs to support TSC scaling, to match the frequency
> of the incoming host.
>

Yes.

> The KVM code for this feature has submitted or integrated?

submitted and can be found at http://www.spinics.net/lists/kvm/msg122431.html

> 
> > Which one(s) you are trying to implement?
> > 
> > In other words, what is the right behavior when KVM_SET_TSC_KHZ fails or
> > KVM_CAP_TSC_CONTROL is not available? We can't answer that question if
> > the requirements and goals are not clear.
> > 
> > Once we know what exactly is the goal, we could enable the new mode with
> > a single option, instead of raw options to control migration stream
> > loading/saving.
> 
> Windows and Linux guests have paravirt clocks and/or options to
> disable direct TSC usage for timekeeping purposes. So disabling
> migration seems overkill.
>

For KVM clock, guest users still need to know the host TSC (possibly
adjusted by scaling and offset) to know how long has passed since the
time provided by the PV clock. The KVM patch has adjusted KVM clock
for VMX TSC scaling so that it can be safely used across migration.

Haozhong

> > 
> > 
> > >  * By default, the migration of vcpu's TSC rate is enabled only on
> > >    pc-*-2.5 and newer machine types. If the cpu option 'save-tsc-freq'
> > >    is present, the vcpu's TSC rate will be migrated from older machine
> > >    types as well.
> > >  * Another cpu option 'load-tsc-freq' controls whether the migrated
> > >    vcpu's TSC rate is used. By default, QEMU will not use the migrated
> > >    TSC rate if this option is not present. Otherwise, QEMU will use
> > >    the migrated TSC rate and override the TSC rate given by the cpu
> > >    option 'tsc-freq'.
> > > 
> > > Changes in v2:
> > >  * Add a pair of cpu options 'save-tsc-freq' and 'load-tsc-freq' to
> > >    control the migration of vcpu's TSC rate.
> > >  * Move all logic of setting TSC rate to target-i386.
> > >  * Remove the duplicated TSC setup in kvm_arch_init_vcpu().
> > > 
> > > Haozhong Zhang (3):
> > >   target-i386: add a subsection for migrating vcpu's TSC rate
> > >   target-i386: calculate vcpu's TSC rate to be migrated
> > >   target-i386: load the migrated vcpu's TSC rate
> > > 
> > >  include/hw/i386/pc.h  |  5 +++++
> > >  target-i386/cpu.c     |  2 ++
> > >  target-i386/cpu.h     |  3 +++
> > >  target-i386/kvm.c     | 61 
> > > +++++++++++++++++++++++++++++++++++++++++++--------
> > >  target-i386/machine.c | 19 ++++++++++++++++
> > >  5 files changed, 81 insertions(+), 9 deletions(-)
> > > 
> > > -- 
> > > 2.4.8
> > > 
> > 
> > -- 
> > Eduardo
>