[PATCH] KVM: PPC: Book3S HV: Validate arch_compat against host compatibility mode

[Amit Machhiwal <amachhiw@linux.ibm.com>]

On IBM POWER systems, newer processor generations can operate in
compatibility modes corresponding to earlier generations. This becomes
relevant for nested virtualization, where nested KVM guests may need to
run with a specific processor compatibility level.

Currently, when running a nested KVM guest (L2) inside a Power11 pSeries
logical partition (L1) booted in Power10 compatibility mode, the guest
fails to boot while setting 'arch_compat'. This happens because the CPU
class is derived from the hardware PVR (via mfspr()), which reflects the
physical processor generation (Power11), rather than the effective
compatibility mode (Power10).

As a result, userspace may request a Power11 arch_compat for the L2
guest. However, the L1 partition, running in Power10 compatibility, has
only negotiated support up to Power10 with the Power Hypervisor (L0).
When H_GUEST_SET_STATE is invoked with a Power11 Logical PVR, the
hypervisor rejects the request, leading to a late guest boot failure:

  KVM-NESTEDv2: couldn't set guest wide elements
  [..KVM reg dump..]

This situation should be detected earlier. Rejecting unsupported
'arch_compat' values in 'kvmppc_set_arch_compat()' avoids issuing an
invalid H_GUEST_SET_STATE hcall and provides a clearer failure mode.

Add a check to reject Power11 'arch_compat' requests when the host is
running in Power10 compatibility mode, returning -EINVAL early instead
of deferring the failure to the hypervisor.

Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Tested-by: Anushree Mathur <anushree.mathur@linux.ibm.com>
Cc: <stable@vger.kernel.org> # v6.13+
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
Changelog:

* Moved this patch out of the v3 series [1] as discussed here [2]
* Addressed below review comments from Ritesh:
  - Based the PVR validation on cpu features
  - Fixed hcall name typo
  - Stable backport

[1] https://lore.kernel.org/all/20260522152744.55251-1-amachhiw@linux.ibm.com/
[2] https://lore.kernel.org/all/20260522152744.55251-2-amachhiw@linux.ibm.com/
---
 arch/powerpc/kvm/book3s_hv.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 61dbeea317f3..e16dbb199366 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -446,7 +446,17 @@ static int kvmppc_set_arch_compat(struct kvm_vcpu *vcpu, u32 arch_compat)
 			guest_pcr_bit = PCR_ARCH_300;
 			break;
 		case PVR_ARCH_31:
+			guest_pcr_bit = PCR_ARCH_31;
+			break;
 		case PVR_ARCH_31_P11:
+			/*
+			 * Need to check this for ISA 3.1, as Power10 and
+			 * Power11 share the same PCR. For any subsequent ISA
+			 * versions, this will be taken care of by the guest vs
+			 * host PCR comparison below.
+			 */
+			if (!cpu_has_feature(CPU_FTR_P11_PVR))
+				return -EINVAL;
 			guest_pcr_bit = PCR_ARCH_31;
 			break;
 		default:

base-commit: ba3e43a9e601636f5edb54e259a74f96ca3b8fd8
-- 
2.50.1 (Apple Git-155)