[PATCH] KVM: arm64: Cap default IPA size to the host's own size

[Will Deacon]

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
> 
> 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.

Is there a way for userspace to discover the default IPA size, or does
it have to try setting values until it finds one that sticks?

Will