On Fri, Feb 15, 2019 at 11:42:27AM +0000, Marc Zyngier wrote: > On Fri, 15 Feb 2019 09:58:57 +0000, > Andre Przywara <andre.przywara at arm.com> wrote: > > > > On Wed, 30 Jan 2019 11:39:00 +0000 > > Andre Przywara <andre.przywara at arm.com> wrote: > > > > Peter, Marc, Christoffer, > > > > can we have an opinion on whether it's useful to introduce some > > common scheme for firmware workaround system registers (parts of > > KVM_REG_ARM_FW_REG(x)), which would allow checking them for > > compatibility between two kernels without specifically knowing about > > them? > > Dave suggested to introduce some kind of signed encoding in the 4 > > LSBs for all those registers (including future ones), where 0 means > > UNKNOWN and greater values are better. So without knowing about the > > particular register, one could judge whether it's safe to migrate. > > I am just not sure how useful this is, given that QEMU seems to ask > > the receiving kernel about any sysreg, and doesn't particularly care > > about the meaning of those registers. And I am not sure we really > > want to introduce some kind of forward looking scheme in the kernel > > here, short of a working crystal ball. I think the kernel policy was > > always to be as strict as possible about those things. > > I honestly don't understand how userspace can decide whether a given > configuration is migratable or not solely based on the value of such a > register. In my experience, the target system has a role to play, and > is the only place where we can find out about whether migration is > actually possible. Both origin and target system need to be taken into account. I don't think that's anything new. > As you said, userspace doesn't interpret the data, nor should it. It > is only on the receiving end that compatibility is assessed and > whether some level of compatibility can be safely ensured. > > So to sum it up, I don't believe in this approach as a general way of > describing the handling or errata. For context, my idea attempted to put KVM, not userspace, in charge of the decision: userspace applies fixed comparison rules determined ahead of time, but KVM supplies the values compared (and hence determines the result). My worry was that otherwise we may end up with a wild-west tangle of arbitrary properties that userspace needs specific knowledge about. We can tolerate a few though. If we accumulate a significant number of errata/vulnerability properties that need to be reported to userspace, this may be worth revisiting. If not, it doesn't matter. Cheers ---Dave
