Re: [Qemu-devel] [PATCH v3 2/3] target-i386: calculate vcpu's TSC rate to be migrated

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Eduardo Habkost (ehabkost@redhat.com) wrote:
> On Mon, Nov 09, 2015 at 08:33:55AM +0800, haozhong.zhang@intel.com wrote:
> > On 11/06/15 13:12, Eduardo Habkost wrote:
> > > On Fri, Nov 06, 2015 at 10:32:24AM +0800, haozhong.zhang@intel.com wrote:
> > > > On 11/05/15 14:05, Eduardo Habkost wrote:
> > > > > On Thu, Nov 05, 2015 at 09:30:51AM +0800, Haozhong Zhang wrote:
> > > > > > On 11/04/15 19:42, Eduardo Habkost wrote:
> > > [...]
> > > > > > > > +        env->tsc_khz_saved = r;
> > > > > > > > +    }
> > > > > > > 
> > > > > > > Why do you need a separate tsc_khz_saved field, and don't simply use
> > > > > > > tsc_khz? It would have the additional feature of letting QMP clients
> > > > > > > query the current TSC rate by asking for the tsc-freq property on CPU
> > > > > > > objects.
> > > > > > >
> > > > > > 
> > > > > > It's to avoid overriding env->tsc_khz on the destination in the
> > > > > > migration. I can change this line to
> > > > > >              env->tsc_khz = env->tsc_khz_saved = r;
> > > > > 
> > > > > You are already avoiding overriding env->tsc_khz, because you use
> > > > > KVM_GET_TSC_KHZ only if tsc_khz is not set yet. I still don't see why
> > > > > you need a tsc_khz_saved field that requires duplicating the SET_TSC_KHZ
> > > > > code, if you could just do this:
> > > > > 
> > > > >     if (!env->tsc_khz) {
> > > > >         env->tsc_khz = kvm_vcpu_ioctl(cs, KVM_GET_TSC_KHZ);
> > > > >     }
> > > > >
> > > > 
> > > > Consider an example that we migrate a VM from machine A to machine B
> > > > and then to machine C, and QEMU on machine B is launched with the cpu
> > > > option 'tsc-freq' (i.e. env->tsc_khz on B is non-zero at the
> > > > beginning):
> > > >  1) In the migration from B to C, the user-specified TSC frequency by
> > > >     'tsc-freq' on B is expected to be migrated to C. That is, the
> > > >     value of env->tsc_khz on B is migrated.
> > > >  2) If TSC frequency is migrated through env->tsc_khz, then
> > > >     env->tsc_khz on B will be overrode in the migration from A to B
> > > >     before kvm_arch_setup_tsc_khz(). If the guest TSC frequency is
> > > >     different than the user-specified TSC frequency on B, the
> > > >     expectation in 1) will not be satisfied anymore.
> > > 
> > > Setting tsc-freq on B when tsc-freq was not used on A is invalid usage.
> > > This is not different from changing the CPU model and adding or removing
> > > CPU flags when migrating, which is also incorrect. The command-line
> > > parameters defining the VM must be the same when you migrate.
> > >
> > 
> > Good to know it's an invalid usage. Then the question is what QEMU is
> > expected to do for this invalid usage?
> > 
> >  1) Abort the migration? But I find that the current QEMU does not
> >     abort the migration between different CPU models (e.g. Nehalem and
> >     Haswell).
> > 
> >  2) Or do not abort the migration and ignore tsc-freq option? If so,
> >     tsc_khz_saved will be not needed.
> 
> My first choice is to abort migration. If we decide to abort today and
> find it to cause problems, we can easily fix it. If we decide to
> continue without aborting, it is difficult to change that behavior
> without breaking existing setups.

Yes, if it's a bad config please abort the migration and put a clear
message in the log so we can tell easily.

Dave

> 
> -- 
> Eduardo
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK