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

[haozhong . zhang]

On 11/06/15 13:12, Eduardo Habkost wrote:
> On Fri, Nov 06, 2015 at 10:32:24AM +0800, address@hidden 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.

Haozhong

> -- 
> Eduardo
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