On Tue, 06 Mar 2018 10:12:42 +0000, peter maydell wrote: > > On 6 March 2018 at 09:50, Marc Zyngier <marc.zyng...@arm.com> wrote: > > On 05/03/18 20:37, Auger Eric wrote: > >> On 05/03/18 17:31, Peter Maydell wrote: > >>> That also means that we will fail migration from a new kernel where > >>> we've specifically asked for PSCI 0.2 to an old PSCI-0.2-only kernel > >>> (because the KVM_REG_ARM_PSCI_VERSION reg will appear in the migration > >>> stream even if its value is the one value that matches the old kernel > >>> behaviour). I don't know if we care about that. > >> > >> Do you know when are we likely to force PSCI 0.2 on a new kernel? At > >> which layer is the decision supposed to be made and on which criteria? > > > > No decent SW should need this. But if you've written a guest that cannot > > work if it doesn't get "2" as response to PSCI_VERSION, you can override it. > > ...but if you want to be able to migrate back from a new kernel to > an old one, then you need to ask the new kernel for 0.2 so it > behaves the same way as the old one. As it stands this code wouldn't > let you do that migration even if you did specifically ask for 0.2. > (As I said, I don't know if we care about that.)
Absolutely. The moment we introduce a new sysreg, we create a migration barrier. I'm not sure how the kernel can help in this respect. M. -- Jazz is not dead, it just smell funny.