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

[Haozhong Zhang]

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.
> 
> Which one(s) you are trying to implement?
>

The former. I agree that it's unpredictable if setting vcpu's TSC
frequency to the migrated value is enabled by default (but not in this
patchset). The cpu option 'load-tsc-freq' is introduced to allow users
to enable this behavior if they do know the underlying KVM and CPU
support TSC scaling. In this way, I think the behavior is predictable
as users do know what they are doing.

> 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.
>

If KVM_CAP_TSC_CONTROL is unavailable, QEMU and KVM will use the host
TSC frequency as vcpu's TSC frequency.

If KVM_CAP_TSC_CONTROL is available and KVM_SET_TSC_KHZ fails, the
setting of TSC frequency will fail and abort either the VM creation
(this is the case for cpu option 'tsc-freq') or the migration.

> 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.
>

Saving vcpu's TSC frequency does not depend on TSC scaling but the
loading does. And that is why I introduce two cpu options to control
them separately.

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