[Qemu-ppc] [RFC for-2.12 7/8] spapr: Handle VMX/VSX presence as an spapr capability flag

[David Gibson]

We currently have some conditionals in the spapr device tree code to decide
whether or not to advertise the availability of the VMX (aka Altivec) and
VSX vector extensions to the guest, based on whether the guest cpu has
those features.

This can lead to confusion and subtle failures on migration, since it makes
a guest visible change based only on host capabilities.  We now have a
better mechanism for this, in spapr capabilities flags, which explicitly
depend on user options rather than host capabilities.

Rework the advertisement of VSX and VMX based on a new VSX capability.  We
no longer bother with a conditional for VMX support, because every CPU
that's ever been supported by the pseries machine type supports VMX.

NOTE: Some userspace distributions (e.g. RHEL7.4) already rely on
availability of VSX in libc, so using cap-vsx=off may lead to a fatal
SIGILL in init.

Signed-off-by: David Gibson <address@hidden>
---
 hw/ppc/spapr.c         | 18 ++++++++++--------
 hw/ppc/spapr_caps.c    | 24 ++++++++++++++++++++++++
 include/hw/ppc/spapr.h |  3 +++
 3 files changed, 37 insertions(+), 8 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index e206825ed9..6b37511e8f 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -556,14 +556,16 @@ static void spapr_populate_cpu_dt(CPUState *cs, void 
*fdt, int offset,
                           segs, sizeof(segs))));
     }
 
-    /* Advertise VMX/VSX (vector extensions) if available
-     *   0 / no property == no vector extensions
+    /* Advertise VSX (vector extensions) if available
      *   1               == VMX / Altivec available
-     *   2               == VSX available */
-    if (env->insns_flags & PPC_ALTIVEC) {
-        uint32_t vmx = (env->insns_flags2 & PPC2_VSX) ? 2 : 1;
-
-        _FDT((fdt_setprop_cell(fdt, offset, "ibm,vmx", vmx)));
+     *   2               == VSX available
+     *
+     * Only CPUs for which we create core types in spapr_cpu_core.c
+     * are possible, and all of those have VMX */
+    if (spapr_has_cap(spapr, SPAPR_CAP_VSX)) {
+        _FDT((fdt_setprop_cell(fdt, offset, "ibm,vmx", 2)));
+    } else {
+        _FDT((fdt_setprop_cell(fdt, offset, "ibm,vmx", 1)));
     }
 
     /* Advertise DFP (Decimal Floating Point) if available
@@ -3829,7 +3831,7 @@ static void spapr_machine_class_init(ObjectClass *oc, 
void *data)
      */
     mc->numa_mem_align_shift = 28;
 
-    smc->default_caps = spapr_caps(SPAPR_CAP_HTM);
+    smc->default_caps = spapr_caps(SPAPR_CAP_HTM | SPAPR_CAP_VSX);
     spapr_capability_properties(smc, &error_abort);
 }
 
diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
index 6bba04d60e..b19a9f4417 100644
--- a/hw/ppc/spapr_caps.c
+++ b/hw/ppc/spapr_caps.c
@@ -44,6 +44,11 @@ static sPAPRCapabilityInfo capability_table[] = {
         .description = "Allow Hardware Transactional Memory (HTM)",
         .bit = SPAPR_CAP_HTM,
     },
+    {
+        .name = "vsx",
+        .description = "Allow Vector Scalar Extensions (VSX)",
+        .bit = SPAPR_CAP_VSX,
+    },
 };
 
 static sPAPRCapabilities default_caps_with_cpu(sPAPRMachineState *spapr, 
CPUState *cs)
@@ -59,6 +64,11 @@ static sPAPRCapabilities 
default_caps_with_cpu(sPAPRMachineState *spapr, CPUStat
         caps.mask &= ~SPAPR_CAP_HTM;
     }
 
+    if (!ppc_check_compat(cpu, CPU_POWERPC_LOGICAL_2_06,
+                          0, spapr->max_compat_pvr)) {
+        caps.mask &= ~SPAPR_CAP_VSX;
+    }
+
     return caps;
 }
 
@@ -149,6 +159,8 @@ const VMStateDescription vmstate_spapr_caps = {
 
 static void spapr_allow_caps(sPAPRMachineState *spapr, Error **errp)
 {
+    PowerPCCPU *cpu = POWERPC_CPU(first_cpu);
+    CPUPPCState *env = &cpu->env;
     Error *err = NULL;
 
     if (spapr_has_cap(spapr, SPAPR_CAP_HTM)) {
@@ -164,6 +176,14 @@ static void spapr_allow_caps(sPAPRMachineState *spapr, 
Error **errp)
         }
     }
 
+    g_assert(env->insns_flags & PPC_ALTIVEC);
+    if (spapr_has_cap(spapr, SPAPR_CAP_VSX)) {
+        if (!(env->insns_flags2 & PPC2_VSX)) {
+            error_setg(&err, "VSX support not available, try cap-vsx=off");
+            goto out;
+        }
+    }
+
 out:
     if (err) {
         error_propagate(errp, err);
@@ -175,6 +195,10 @@ static void spapr_enforce_caps(sPAPRMachineState *spapr, 
Error **errp)
     if (!spapr_has_cap(spapr, SPAPR_CAP_HTM)) {
         /* TODO: Tell KVM not to allow HTM instructions */
     }
+
+    if (!spapr_has_cap(spapr, SPAPR_CAP_VSX)) {
+        /* TODO: Any way to disable VSX? */
+    }
 }
 
 void spapr_caps_reset(sPAPRMachineState *spapr)
diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
index 9b80fe72ed..4c173cb40c 100644
--- a/include/hw/ppc/spapr.h
+++ b/include/hw/ppc/spapr.h
@@ -59,6 +59,9 @@ typedef enum {
 /* Hardware Transactional Memory */
 #define SPAPR_CAP_HTM               0x0000000000000001ULL
 
+/* Vector Scalar Extensions */
+#define SPAPR_CAP_VSX               0x0000000000000002ULL
+
 typedef struct sPAPRCapabilities sPAPRCapabilities;
 struct sPAPRCapabilities {
     uint64_t mask;
-- 
2.14.3