[Qemu-devel] [QEMU-PPC] [PATCH v3] powerpc/spapr: Add host threads parameter to ibm, get_system_parameter

[Suraj Jitindar Singh <sjitindarsingh@gmail.com>]

The ibm,get_system_parameter rtas call is used by the guest to retrieve
data relating to certain parameters of the system. The SPLPAR
characteristics option (token 20) is used to determin characteristics of
the environment in which the lpar will run.

It may be useful for a guest to know the number of physical host threads
present on the underlying system where it is being run. Add the
characteristic "HostThrs" to the SPLPAR Characteristics
ibm,get_system_parameter rtas call to expose this information to a
guest and provide an implementation which determines this information
based on the number of interrupt servers present in the device tree.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>

---

V1 -> V2:
- Take into account that the core may be operating in split core mode
  meaning a single core may be split into multiple subcores.
V2 -> V3:
- Add curly braces for single line if statements
---
 hw/ppc/spapr_rtas.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/hw/ppc/spapr_rtas.c b/hw/ppc/spapr_rtas.c
index 5bc1a93271..1bab71c90c 100644
--- a/hw/ppc/spapr_rtas.c
+++ b/hw/ppc/spapr_rtas.c
@@ -229,6 +229,58 @@ static inline int sysparm_st(target_ulong addr, target_ulong len,
     return RTAS_OUT_SUCCESS;
 }
 
+#define CPUS_PATH       "/proc/device-tree/cpus/"
+#define SUBCORE_PATH    "/sys/devices/system/cpu/subcores_per_core"
+
+static int rtas_get_num_host_threads(void)
+{
+    int num_threads = -1;
+    unsigned long len;
+    const char *entry;
+    char *buf;
+    GDir *dir;
+
+    if (!kvm_enabled()) {
+        return 1;
+    }
+
+    /* Read interrupt servers to determine number of threads per core */
+    dir = g_dir_open(CPUS_PATH, 0, NULL);
+    if (!dir) {
+        return -1;
+    }
+
+    while ((entry = g_dir_read_name(dir))) {
+        if (!strncmp(entry, "PowerPC,POWER", strlen("PowerPC,POWER"))) {
+            char *path;
+
+            path = g_strconcat(CPUS_PATH, entry, "/ibm,ppc-interrupt-server#s",
+                               NULL);
+            if (g_file_get_contents(path, &buf, &len, NULL)) {
+                num_threads = len / sizeof(int);
+                g_free(buf);
+            }
+
+            g_free(path);
+            break;
+        }
+    }
+
+    g_dir_close(dir);
+
+    /* Check if split core mode in use */
+    if (g_file_get_contents(SUBCORE_PATH, &buf, &len, NULL)) {
+        int subcores = g_ascii_strtoll(buf, NULL, 10);
+
+        if (subcores) {
+            num_threads /= subcores;
+        }
+        g_free(buf);
+    }
+
+    return num_threads;
+}
+
 static void rtas_ibm_get_system_parameter(PowerPCCPU *cpu,
                                           SpaprMachineState *spapr,
                                           uint32_t token, uint32_t nargs,
@@ -250,6 +302,16 @@ static void rtas_ibm_get_system_parameter(PowerPCCPU *cpu,
                                           current_machine->ram_size / MiB,
                                           smp_cpus,
                                           max_cpus);
+        int num_host_threads = rtas_get_num_host_threads();
+
+        if (num_host_threads > 0) {
+            char *hostthr_val, *old = param_val;
+
+            hostthr_val = g_strdup_printf(",HostThrs=%d", num_host_threads);
+            param_val = g_strconcat(param_val, hostthr_val, NULL);
+            g_free(hostthr_val);
+            g_free(old);
+        }
         ret = sysparm_st(buffer, length, param_val, strlen(param_val) + 1);
         g_free(param_val);
         break;
-- 
2.13.6