[PATCH 2/2] target/ppc/kvm: Use KVM_CAP_PPC_AIL_MODE_3 to determine cap-ail-mode-3 support

[Nicholas Piggin <npiggin@gmail.com>]

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 <npiggin@gmail.com>
---
 hw/ppc/spapr_caps.c       |  2 +-
 linux-headers/linux/kvm.h |  1 +
 target/ppc/kvm.c          | 18 +++++++++++++++++-
 target/ppc/kvm_ppc.h      |  4 ++--
 4 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
index 5fd4a53c33..5cc80776d0 100644
--- a/hw/ppc/spapr_caps.c
+++ b/hw/ppc/spapr_caps.c
@@ -619,7 +619,7 @@ static void cap_ail_mode_3_apply(SpaprMachineState *spapr,
     ERRP_GUARD();
 
     if (kvm_enabled()) {
-        if (!kvmppc_supports_ail_3()) {
+        if (!kvmppc_has_cap_ail_3()) {
             error_setg(errp, "KVM implementation does not support cap-ail-mode-3");
             error_append_hint(errp, "Try appending -machine cap-ail-mode-3=off\n");
             return;
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 128bc530d4..d0d0bdaac4 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;
@@ -2563,10 +2565,24 @@ int kvmppc_has_cap_rpt_invalidate(void)
     return cap_rpt_invalidate;
 }
 
-int kvmppc_supports_ail_3(void)
+int kvmppc_has_cap_ail_3(void)
 {
     PowerPCCPUClass *pcc = kvm_ppc_get_host_cpu_class();
 
+    if (cap_ail_mode_3) {
+        return 1;
+    }
+
+    if (kvm_ioctl(kvm_state, KVM_CHECK_EXTENSION, KVM_CAP_PPC_AIL_MODE_3) == 0) {
+        return 0;
+    }
+
+    /*
+     * For KVM hosts that pre-date this cap, special-case the test because
+     * the performance cost for disabling the feature unconditionally is
+     * prohibitive.
+     */
+
     /*
      * KVM PR only supports AIL-0
      */
diff --git a/target/ppc/kvm_ppc.h b/target/ppc/kvm_ppc.h
index d9d1c54955..efafa67b83 100644
--- a/target/ppc/kvm_ppc.h
+++ b/target/ppc/kvm_ppc.h
@@ -73,7 +73,7 @@ int kvmppc_set_cap_nested_kvm_hv(int enable);
 int kvmppc_get_cap_large_decr(void);
 int kvmppc_enable_cap_large_decr(PowerPCCPU *cpu, int enable);
 int kvmppc_has_cap_rpt_invalidate(void);
-int kvmppc_supports_ail_3(void);
+int kvmppc_has_cap_ail_3(void);
 int kvmppc_enable_hwrng(void);
 int kvmppc_put_books_sregs(PowerPCCPU *cpu);
 PowerPCCPUClass *kvm_ppc_get_host_cpu_class(void);
@@ -394,7 +394,7 @@ static inline int kvmppc_has_cap_rpt_invalidate(void)
     return false;
 }
 
-static inline int kvmppc_supports_ail_3(void)
+static inline int kvmppc_has_cap_ail_3(void)
 {
     return false;
 }
-- 
2.23.0