Hi Andrew, On Tue, 09 Mar 2021 13:20:21 +0000, Andrew Jones <drjones@xxxxxxxxxx> wrote: > > Hi Marc, > > On Mon, Mar 08, 2021 at 05:46:43PM +0000, Marc Zyngier wrote: > > KVM/arm64 has forever used a 40bit default IPA space, partially > > due to its 32bit heritage (where the only choice is 40bit). > > > > However, there are implementations in the wild that have a *cough* > > much smaller *cough* IPA space, which leads to a misprogramming of > > VTCR_EL2, and a guest that is stuck on its first memory access > > if userspace dares to ask for the default IPA setting (which most > > VMMs do). > > > > Instead, cap the default IPA size to what the host can actually > > do, and spit out a one-off message on the console. The boot warning > > is turned into a more meaningfull message, and the new behaviour > > is also documented. > > > > Although this is a userspace ABI change, it doesn't really change > > much for userspace: > > > > - the guest couldn't run before this change, while it now has > > a chance to if the memory range fits the reduced IPA space > > > > - a memory slot that was accepted because it did fit the default > > IPA space but didn't fit the HW constraints is now properly > > rejected > > I'm not sure deferring the misconfiguration error until memslot > request time is better than just failing to create a VM. If > userspace doesn't use KVM_CAP_ARM_VM_IPA_SIZE to determine the > limit (which it hasn't been obliged to do) and it is able to > successfully create a VM, then it will assume up to 40-bit IPAs > are supported. Later, when it tries to add memslots and fails > it may be confused, especially if that later is much, much later > with memory hotplug. That's a fair point. However, no existing userspace will work on these systems. Is that what we want to do? I don't care much, but having non-usable defaults feel a bit... odd. I do spit out a warning, but I agree this isn't great either. > > The other thing that's left doing is to convince userspace to > > actually use the IPA space setting instead of relying on the > > antiquated default. > > Failing to create any VM which hasn't selected a valid IPA limit > should be pretty convincing :-) I'll make sure to redirect the reports your way! :D Thanks, M. -- Without deviation from the norm, progress is not possible.
