On Tue, Jul 21, 2020 at 03:04:45PM +1000, Paul Mackerras wrote:
> On Tue, Jun 23, 2020 at 06:50:27PM +0200, Cédric Le Goater wrote:
> > The TIDR register is only available on POWER9 systems and code
> > accessing this register is not always protected by the CPU_FTR_P9_TIDR
> > flag. Fix that to make sure POWER10 systems won't use it as TIDR has
> > been removed.
> 
> I'm concerned about what this patch would do if we are trying to
> migrate from a P9 guest to a guest on P10 in P9-compat mode, in that
> the destination QEMU would get an error on doing the SET_ONE_REG for
> the TIDR.  I don't think the lack of TIDR is worth failing the
> migration for given that TIDR only actually does anything if you are
> using an accelerator, and KVM has never supported use of accelerators
> in guests.  I'm cc'ing David Gibson for his comments on the
> compatibility and migration issues.

Having thought about this a bit more, I don't think it matters.  We're
going to have to update qemu to handle POWER10 anyway.  If this causes
a problem, this would just add one small thing to whatever we need to
fix there.

> In any case, given that both move to and move from TIDR will be no-ops
> on P10 (for privileged code), I don't think there is a great urgency
> for this patch.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson