On Sat, 10 Nov 2018 22:18:47 +0000, Manish Jaggi <mjaggi@xxxxxxxxxxxxxxxxxx> wrote: > > > CCing a larger audience. > Please review. > > On 10/23/2018 03:51 PM, Jaggi, Manish wrote: > > From: Manish Jaggi <manish.jaggi@xxxxxxxxxx> > > > > This patch introduces an error code KVM_EINVARIANT which is returned > > by KVM when userland tries to set an invariant register. > > > > The need for this error code is in VM Migration for arm64. > > ARM64 systems use mainly -machine virt -cpu host as parameter to qemu. > > Migration requires both Source and destination machines to have same > > physical cpu. There are cases where the overall architecture of CPU is > > same but the next version of the chip with some bug fixes which have no > > effect on qemu operation. In such cases invariant registers like MIDR > > have a different value. > > Currently Migration fails in such cases. > > > > Rather than sending a EINVAL, a specifc error code will help > > userland program the guest invariant register by querying the migrated > > host machines invariant registers. But failing migration is a good thing, right? How do you expect that the guest will be happy to see a new CPU revision right in the middle of its execution? Do you also propose that QEMU starts doing that for big-little systems? After all, if ignoring the differences in some registers is harmless for migration, surely that should be the case in a single system, right? > > > > Qemu will have a parameter -hostinvariant along with checking of this > > error code. So it can be safely assumed that the feature is opt-in You're changing the ABI without any buy in from userspace, which is not acceptable. As it stands, this patch creates a number of issues without solving any. Things to think about: - How does errata management works when migrating to a different system? - big-little, as mentioned above - Are all invariant registers equal? A different MIDR has the same effect as a different MMFR0? Instead of papering over architectural constants i a system, how about allowing the relevant ID registers to be overloaded when not incompatible? Thanks, M. -- Jazz is not dead, it just smell funny. _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
