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

[Marc Zyngier <maz@kernel.org>]

Hi Andrew,

On Tue, 09 Mar 2021 13:20:21 +0000,
Andrew Jones <drjones@redhat.com> 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.
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