Re: [Qemu-devel] [PATCH v4 2/3] target-i386: add migration support for Intel LMCE

[Eduardo Habkost]

On Thu, Jun 16, 2016 at 07:40:20PM +0200, Paolo Bonzini wrote:
> 
> 
> On 16/06/2016 19:36, Eduardo Habkost wrote:
> >> > 
> >> > Eduardo said nice for this part in previous version [1], so we may wait
> >> > for his comments?
> >> > 
> >> > [1] http://lists.nongnu.org/archive/html/qemu-devel/2016-06/msg01992.html
> > I agree we don't need this check, but I still believe it is a
> > nice thing to have.
> > 
> > In addition to detecting user errors, they don't hurt and are
> > useful for things like "-cpu host", that don't guarantee
> > live-migration compatibility but still allow migration if you
> > ensure host capabilities are the same on both sides.
> 
> On the other hand we don't check for this on any other property, either
> CPU or device, do we?  Considering "lmce=on" always breaks on an old
> kernel (i.e. there's no need for an explicit ",enforce" on the -cpu
> flag), I think it's unnecessary and makes things inconsistent.

BTW, just found another case where we check for migration mismatches: TSC
frequency.

    if (env->tsc_khz && env->user_tsc_khz &&
        env->tsc_khz != env->user_tsc_khz) {
        error_report("Mismatch between user-specified TSC frequency and "
                     "migrated TSC frequency");
        return -EINVAL;
    }

We can do that because tsc_khz is unusual like mcg_cap: it can be
configured by the user but is also included in the migration
stream.

-- 
Eduardo