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KVM: arm64: Allow userspace to change the implementation ID registers

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KVM's treatment of the ID registers that describe the implementation
(MIDR, REVIDR, and AIDR) is interesting, to say the least. On the
userspace-facing end of it, KVM presents the values of the boot CPU on
all vCPUs and treats them as invariant. On the guest side of things KVM
presents the hardware values of the local CPU, which can change during
CPU migration in a big-little system.

While one may call this fragile, there is at least some degree of
predictability around it. For example, if a VMM wanted to present
big-little to a guest, it could affine vCPUs accordingly to the correct
clusters.

All of this makes a giant mess out of adding support for making these
implementation ID registers writable. Avoid breaking the rather subtle
ABI around the old way of doing things by requiring opt-in from
userspace to make the registers writable.

When the cap is enabled, allow userspace to set MIDR, REVIDR, and AIDR
to any non-reserved value and present those values consistently across
all vCPUs.

Signed-off-by: Sebastian Ott <sebott@redhat.com>
[oliver: changelog, capability]
Link: https://lore.kernel.org/r/20250225005401.679536-5-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
This commit is contained in:
Sebastian Ott
2025-02-24 16:54:00 -08:00
committed by Oliver Upton
parent d0d81e03e6
commit 3adaee7830
6 changed files with 78 additions and 7 deletions
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18
Documentation/virt/kvm/api.rst
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@@ -8258,6 +8258,24 @@ KVM exits with the register state of either the L1 or L2 guest
depending on which executed at the time of an exit. Userspace must
take care to differentiate between these cases.
7.37 KVM_CAP_ARM_WRITABLE_IMP_ID_REGS
-------------------------------------
:Architectures: arm64
:Target: VM
:Parameters: None
:Returns: 0 on success, -EBUSY if vCPUs have been created before enabling this
capability.
This capability changes the behavior of the registers that identify a PE
implementation of the Arm architecture: MIDR_EL1, REVIDR_EL1, and AIDR_EL1.
By default, these registers are visible to userspace but treated as invariant.
When this capability is enabled, KVM allows userspace to change the
aforementioned registers before the first KVM_RUN. These registers are VM
scoped, meaning that the same set of values are presented on all vCPUs in a
given VM.
8. Other capabilities.
======================