[Qemu-devel] [PATCH 1/2] target/ppc: Clarify compat mode max_threads value

[David Gibson <david@gibson.dropbear.id.au>]

We recently had some discussions that were sidetracked for a while, because
nearly everyone misapprehended the purpose of the 'max_threads' field in
the compatiblity modes table.  It's all about guest expectations, not host
expectations or support (that's handled elsewhere).

In an attempt to avoid a repeat of that confusion, rename the field to
'max_vthreads' and add an explanatory comment.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 hw/ppc/spapr.c      |  4 ++--
 target/ppc/compat.c | 25 +++++++++++++++++--------
 target/ppc/cpu.h    |  2 +-
 3 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 3e528fe91e..e35214bfc3 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -345,7 +345,7 @@ static int spapr_fixup_cpu_dt(void *fdt, sPAPRMachineState *spapr)
         PowerPCCPU *cpu = POWERPC_CPU(cs);
         DeviceClass *dc = DEVICE_GET_CLASS(cs);
         int index = spapr_vcpu_id(cpu);
-        int compat_smt = MIN(smp_threads, ppc_compat_max_threads(cpu));
+        int compat_smt = MIN(smp_threads, ppc_compat_max_vthreads(cpu));
 
         if ((index % smt) != 0) {
             continue;
@@ -503,7 +503,7 @@ static void spapr_populate_cpu_dt(CPUState *cs, void *fdt, int offset,
     size_t page_sizes_prop_size;
     uint32_t vcpus_per_socket = smp_threads * smp_cores;
     uint32_t pft_size_prop[] = {0, cpu_to_be32(spapr->htab_shift)};
-    int compat_smt = MIN(smp_threads, ppc_compat_max_threads(cpu));
+    int compat_smt = MIN(smp_threads, ppc_compat_max_vthreads(cpu));
     sPAPRDRConnector *drc;
     int drc_index;
     uint32_t radix_AP_encodings[PPC_PAGE_SIZES_MAX_SZ];
diff --git a/target/ppc/compat.c b/target/ppc/compat.c
index 276b5b52c2..807c906f68 100644
--- a/target/ppc/compat.c
+++ b/target/ppc/compat.c
@@ -32,7 +32,16 @@ typedef struct {
     uint32_t pvr;
     uint64_t pcr;
     uint64_t pcr_level;
-    int max_threads;
+
+    /*
+     * Maximum allowed virtual threads per virtual core
+     *
+     * This is to stop older guests getting confused by seeing more
+     * threads than they think the cpu can support.  Usually it's
+     * equal to the number of threads supported on bare metal
+     * hardware, but not always (see POWER9).
+     */
+    int max_vthreads;
 } CompatInfo;
 
 static const CompatInfo compat_table[] = {
@@ -45,28 +54,28 @@ static const CompatInfo compat_table[] = {
         .pcr = PCR_COMPAT_3_00 | PCR_COMPAT_2_07 | PCR_COMPAT_2_06 |
                PCR_COMPAT_2_05 | PCR_TM_DIS | PCR_VSX_DIS,
         .pcr_level = PCR_COMPAT_2_05,
-        .max_threads = 2,
+        .max_vthreads = 2,
     },
     { /* POWER7, ISA2.06 */
         .name = "power7",
         .pvr = CPU_POWERPC_LOGICAL_2_06,
         .pcr = PCR_COMPAT_3_00 | PCR_COMPAT_2_07 | PCR_COMPAT_2_06 | PCR_TM_DIS,
         .pcr_level = PCR_COMPAT_2_06,
-        .max_threads = 4,
+        .max_vthreads = 4,
     },
     {
         .name = "power7+",
         .pvr = CPU_POWERPC_LOGICAL_2_06_PLUS,
         .pcr = PCR_COMPAT_3_00 | PCR_COMPAT_2_07 | PCR_COMPAT_2_06 | PCR_TM_DIS,
         .pcr_level = PCR_COMPAT_2_06,
-        .max_threads = 4,
+        .max_vthreads = 4,
     },
     { /* POWER8, ISA2.07 */
         .name = "power8",
         .pvr = CPU_POWERPC_LOGICAL_2_07,
         .pcr = PCR_COMPAT_3_00 | PCR_COMPAT_2_07,
         .pcr_level = PCR_COMPAT_2_07,
-        .max_threads = 8,
+        .max_vthreads = 8,
     },
     { /* POWER9, ISA3.00 */
         .name = "power9",
@@ -80,7 +89,7 @@ static const CompatInfo compat_table[] = {
          * confusing if half of the threads disappear from the guest
          * if it announces it's POWER9 aware at CAS time.
          */
-        .max_threads = 8,
+        .max_vthreads = 8,
     },
 };
 
@@ -192,14 +201,14 @@ void ppc_set_compat_all(uint32_t compat_pvr, Error **errp)
     }
 }
 
-int ppc_compat_max_threads(PowerPCCPU *cpu)
+int ppc_compat_max_vthreads(PowerPCCPU *cpu)
 {
     const CompatInfo *compat = compat_by_pvr(cpu->compat_pvr);
     int n_threads = CPU(cpu)->nr_threads;
 
     if (cpu->compat_pvr) {
         g_assert(compat);
-        n_threads = MIN(n_threads, compat->max_threads);
+        n_threads = MIN(n_threads, compat->max_vthreads);
     }
 
     return n_threads;
diff --git a/target/ppc/cpu.h b/target/ppc/cpu.h
index a5e49f23e9..dc6820c5eb 100644
--- a/target/ppc/cpu.h
+++ b/target/ppc/cpu.h
@@ -1395,7 +1395,7 @@ void ppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr, Error **errp);
 #if !defined(CONFIG_USER_ONLY)
 void ppc_set_compat_all(uint32_t compat_pvr, Error **errp);
 #endif
-int ppc_compat_max_threads(PowerPCCPU *cpu);
+int ppc_compat_max_vthreads(PowerPCCPU *cpu);
 void ppc_compat_add_property(Object *obj, const char *name,
                              uint32_t *compat_pvr, const char *basedesc,
                              Error **errp);
-- 
2.14.3