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

On Tue, Mar 09, 2021 at 01:43:40PM +0000, Marc Zyngier wrote:

Hi Andrew,

On Tue, 09 Mar 2021 13:20:21 +0000,
Andrew Jones [off-list ref] wrote:

quoted

Hi Marc,

On Mon, Mar 08, 2021 at 05:46:43PM +0000, Marc Zyngier wrote:

quoted

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.

I can send patches for QEMU, KVM selftests, and maybe even rust-vmm.
Can you point me to something about these systems I can reference
in my postings? Or I can just reference this mail thread.

quoted

quoted

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

What's the current error message when this occurs? Is it good enough, or
should we improve it to help provide people hints? Please don't change
it to "Invalid IPA limit, please mail Andrew Jones" :-)

Thanks,
drew


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