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. > > 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 :-) Thanks, drew
