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
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