Hi Suzuki,

On Tue, 09 Mar 2021 11:09:48 +0000,
Suzuki Poulose [off-list ref] wrote:

quoted

On 8 Mar 2021, at 17:46, Marc Zyngier [off-list ref] 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.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>

Thanks for that. Whilst I have your attention and given that you are
responsible for most of the variable IPA stuff... ;-)

I think we have another issue around the handling of our IPA
size. Let's say I create a VM with a 32bit IPA space. If I register a
2GB memslot at 0x8000000, I'm getting an error, which I think is
bogus.

I came to the conclusion that kvm_arch_prepare_memory_region() is a
bit overzealous when rejecting the memslot, and I used the following
patchlet to address it.

Does this seem sensible to you?

	M.

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 77cb2d28f2a4..8711894db8c2 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c

@@ -1312,8 +1312,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 	 * Prevent userspace from creating a memory region outside of the IPA
 	 * space addressable by the KVM guest IPA space.
 	 */
-	if (memslot->base_gfn + memslot->npages >=
-	    (kvm_phys_size(kvm) >> PAGE_SHIFT))
+	if ((memslot->base_gfn + memslot->npages) > (kvm_phys_size(kvm) >> PAGE_SHIFT))
 		return -EFAULT;
 
 	mmap_read_lock(current->mm);

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

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