target/ppc: Clarify compat mode max_threads value

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>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>

1 files changed, 17 insertions, 8 deletions

diff --git a/target/ppc/compat.c b/target/ppc/compat.c
index 276b5b5..807c906 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;