[PATCH 4/6] KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM

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

On POWER systems, the host CPU may run in a compatibility mode (e.g., a
Power11 processor operating in Power10 compatibility mode). In such
cases, the effective CPU level exposed to guests differs from the
physical processor generation.

When running nested KVM guests, QEMU derives the host CPU type using
mfpvr(), which reflects the physical processor version. This can result
in a mismatch between the CPU model selected by QEMU and the
compatibility mode enforced by the host, leading to guest boot failures.

For example, booting a nested guest on a Power11 LPAR configured in
Power10 compatibility mode fails with:

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

This occurs because QEMU selects a CPU model corresponding to the
physical processor (via mfpvr()), while the host operates in a lower
compatibility mode. As a result, KVM rejects the requested compatibility
level during guest initialization.

Add support for retrieving host CPU compatibility capabilities for
nested guests on PowerVM (PAPR nested API v2). The hypervisor provides
the effective compatibility levels via the H_GUEST_GET_CAPABILITIES
hcall, which reflects the processor modes negotiated between the Power
hypervisor (L0) and the host partition (L1).

On pseries systems, obtain the capability bitmap using
plpar_guest_get_capabilities() and return it via struct
kvm_ppc_compat_caps. This information is then exposed to userspace
through the KVM_PPC_GET_COMPAT_CAPS ioctl.

Hook the implementation into the Book3S HV kvmppc_ops so that it can be
invoked by the generic KVM ioctl handling code.

Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
 arch/powerpc/kvm/book3s_hv.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 948c6b099a29..d602d90111d1 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -6516,6 +6516,22 @@ static bool kvmppc_hash_v3_possible(void)
 	return true;
 }
 
+
+static int kvmppc_get_compat_cpu_caps(struct kvm_ppc_compat_caps *host_caps)
+{
+
+	unsigned long capabilities = 0;
+	long rc = -EINVAL;
+
+	if (kvmhv_on_pseries()) {
+		if (kvmhv_is_nestedv2())
+			rc = plpar_guest_get_capabilities(0, &capabilities);
+		host_caps->compat_capabilities = capabilities;
+	}
+
+	return rc;
+}
+
 static struct kvmppc_ops kvm_ops_hv = {
 	.get_sregs = kvm_arch_vcpu_ioctl_get_sregs_hv,
 	.set_sregs = kvm_arch_vcpu_ioctl_set_sregs_hv,
@@ -6558,6 +6574,7 @@ static struct kvmppc_ops kvm_ops_hv = {
 	.hash_v3_possible = kvmppc_hash_v3_possible,
 	.create_vcpu_debugfs = kvmppc_arch_create_vcpu_debugfs_hv,
 	.create_vm_debugfs = kvmppc_arch_create_vm_debugfs_hv,
+	.get_compat_cpu_ver = kvmppc_get_compat_cpu_caps,
 };
 
 static int kvm_init_subcore_bitmap(void)
-- 
2.50.1