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

[Will Deacon]

On Tue, Mar 09, 2021 at 11:35:54AM +0000, Marc Zyngier wrote:
> 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).

Great, thanks -- that's exactly what I was thinking about when I asked the
question!

Will