[PATCH v2 3/3] target/ppc/kvm: Use KVM_CAP_PPC_AIL_MODE_3 to determine cap-ail-mode-3 support

[Nicholas Piggin]

Use KVM_CAP_PPC_AIL_MODE_3 to determine cap-ail-mode-3 support for KVM
guests. Keep the fallback heuristic for KVM hosts that pre-date this
CAP.

This is only proposed the KVM CAP has not yet been allocated. I will
ask to merge the new KVM cap when there are no objections on the QEMU
side.

not-yet-Signed-off-by: Nicholas Piggin <npig...@gmail.com>
---
Since v1: 
- Remove incorrect test for unsupported cap query. Add comment instead.

 linux-headers/linux/kvm.h |  1 +
 target/ppc/kvm.c          | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/linux-headers/linux/kvm.h b/linux-headers/linux/kvm.h
index 02c5e7b7bb..d91f578200 100644
--- a/linux-headers/linux/kvm.h
+++ b/linux-headers/linux/kvm.h
@@ -1130,6 +1130,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_BINARY_STATS_FD 203
 #define KVM_CAP_EXIT_ON_EMULATION_FAILURE 204
 #define KVM_CAP_ARM_MTE 205
+#define KVM_CAP_PPC_AIL_MODE_3 210
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
index 1338c41f8f..7f71f78253 100644
--- a/target/ppc/kvm.c
+++ b/target/ppc/kvm.c
@@ -90,6 +90,7 @@ static int cap_ppc_nested_kvm_hv;
 static int cap_large_decr;
 static int cap_fwnmi;
 static int cap_rpt_invalidate;
+static int cap_ail_mode_3;
 
 static uint32_t debug_inst_opcode;
 
@@ -154,6 +155,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
     }
 
     cap_rpt_invalidate = kvm_vm_check_extension(s, KVM_CAP_PPC_RPT_INVALIDATE);
+    cap_ail_mode_3 = kvm_vm_check_extension(s, KVM_CAP_PPC_AIL_MODE_3);
     kvm_ppc_register_host_cpu_type();
 
     return 0;
@@ -2567,6 +2569,17 @@ bool kvmppc_supports_ail_3(void)
 {
     PowerPCCPUClass *pcc = kvm_ppc_get_host_cpu_class();
 
+    if (cap_ail_mode_3) {
+        return 1;
+    }
+
+    /*
+     * cap-ail-mode-3 is disabled, but it may be because the KVM host pre-dates
+     * the cap. Special-case the test because the performance cost for
+     * disabling the feature unconditionally is prohibitive until updated
+     * KVM is widely in use.
+     */
+
     /*
      * KVM PR only supports AIL-0
      */
@@ -2589,6 +2602,11 @@ bool kvmppc_supports_ail_3(void)
         return 0;
     }
 
+    /*
+     * Beyond ISA v3.1 (POWER10), this could return 0, because all KVM
+     * implementations for such hosts would support the cap.
+     */
+
     return 1;
 }
 
-- 
2.23.0