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

[Marc Zyngier]

On Tue, 09 Mar 2021 11:26:59 +0000,
Will Deacon <will at kernel.org> wrote:
> 
> 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?

Yes, since 233a7cb23531 ("kvm: arm64: Allow tuning the physical
address size for VM").

The VMM can issue a KVM_CAP_ARM_VM_IPA_SIZE ioctl(), and get in return
the maximum IPA size (I have a patch for kvmtool that does this).

	M.

-- 
Without deviation from the norm, progress is not possible.