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KVM: arm64: Reject VM creation when the default IPA size is unsupported

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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, blundly reject the creation of such VM, as we can't
satisfy the requirements from userspace (with a one-off warning).
Also clarify the boot warning, and document that the VM creation
will fail when an unsupported IPA size is provided.

Although this is an ABI change, it doesn't really change much
for userspace:

- the guest couldn't run before this change, but no error was
  returned. At least userspace knows what is happening.

- a memory slot that was accepted because it did fit the default
  IPA space now doesn't even get a chance to be registered.

The other thing that is left doing is to convince userspace to
actually use the IPA space setting instead of relying on the
antiquated default.

Fixes: 233a7cb235 ("kvm: arm64: Allow tuning the physical address size for VM")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Link: https://lore.kernel.org/r/20210311100016.3830038-2-maz@kernel.org
This commit is contained in:
Marc Zyngier 2021-03-11 10:00:15 +00:00
parent 01dc9262ff
commit 7d717558dd
2 changed files with 11 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
Documentation/virt/kvm/api.rst
View File

@ -182,6 +182,9 @@ is dependent on the CPU capability and the kernel configuration. The limit can
be retrieved using KVM_CAP_ARM_VM_IPA_SIZE of the KVM_CHECK_EXTENSION
ioctl() at run-time.
Creation of the VM will fail if the requested IPA size (whether it is
implicit or explicit) is unsupported on the host.
Please note that configuring the IPA size does not affect the capability
exposed by the guest CPUs in ID_AA64MMFR0_EL1[PARange]. It only affects
size of the address translated by the stage2 level (guest physical to

12
arch/arm64/kvm/reset.c
View File

@ -324,10 +324,9 @@ int kvm_set_ipa_limit(void)
}
kvm_ipa_limit = id_aa64mmfr0_parange_to_phys_shift(parange);
WARN(kvm_ipa_limit < KVM_PHYS_SHIFT,
"KVM IPA Size Limit (%d bits) is smaller than default size\n",
kvm_ipa_limit);
kvm_info("IPA Size Limit: %d bits\n", kvm_ipa_limit);
kvm_info("IPA Size Limit: %d bits%s\n", kvm_ipa_limit,
((kvm_ipa_limit < KVM_PHYS_SHIFT) ?
" (Reduced IPA size, limited VM/VMM compatibility)" : ""));
return 0;
}
@ -356,6 +355,11 @@ int kvm_arm_setup_stage2(struct kvm *kvm, unsigned long type)
return -EINVAL;
} else {
phys_shift = KVM_PHYS_SHIFT;
if (phys_shift > kvm_ipa_limit) {
pr_warn_once("%s using unsupported default IPA limit, upgrade your VMM\n",
current->comm);
return -EINVAL;
}
}
mmfr0 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR0_EL1);