On 18/02/2019 09:07, Marc Zyngier wrote: > On Fri, 15 Feb 2019 17:26:02 +0000 > Dave Martin <Dave.Martin@xxxxxxx> wrote: Hi, > >> 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@xxxxxxx> wrote: >>>> >>>> On Wed, 30 Jan 2019 11:39:00 +0000 >>>> Andre Przywara <andre.przywara@xxxxxxx> 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. > > Well, that was what I understood from Andre's question. > >> >>> 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. > > And this is where our understanding differs. I do not think userspace > has to care at all. All it has to do is to provide the saved register > values to the target system, and let KVM accept or refuse these > settings. I can't see what providing a set of predefined values back to > userspace gains us. > > An unknown register on the target system fails the restore phase: > that's absolutely fine, as we don't want to run on a system that > doesn't know about the mitigation. > > An incompatible value fails the restore as well, as KVM itself finds > that this is a service it cannot safely provide. > > No userspace involvement, no QEMU upgrade required. Only the kernel > knows about it. Yes, this is what I understand as well. From experience, many times when we were not strict enough about some userland interface, it backfired. The only case where such a forward-looking scheme would make sense is the case where the source system has a new kernel, advertising a new firmware workaround register, in an unknown or missing state (0 or -1). An older kernel on the target system might not know about this register. That would translate into "unknown", which is compatible with 0 or -1 from the source. So migration would be fine, but we deny it because the new kernel returns -EINVAL. But I am not sure this construct is worth implementing in the kernel. If people care about this case, they could implement a workaround in userland instead. Or just upgrade the target kernel before migration. >> 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. > > Andre: if you want this to make it into 5.1, the time is now. OK. So is v2 [1] fine then? This implements the much easier "bigger is better" scheme, but being 0 based instead of using a 4-bit signed encoding. Let me know if there is something to rework in there. Cheers, Andre. [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2019-January/627739.html
