[Qemu-devel] [PULL v2 04/14] target-i386: present virtual L3 cache info for vcpus

[Michael S. Tsirkin]

From: "Longpeng(Mike)" <address@hidden>

Some software algorithms are based on the hardware's cache info, for example,
for x86 linux kernel, when cpu1 want to wakeup a task on cpu2, cpu1 will trigger
a resched IPI and told cpu2 to do the wakeup if they don't share low level
cache. Oppositely, cpu1 will access cpu2's runqueue directly if they share llc.
The relevant linux-kernel code as bellow:

        static void ttwu_queue(struct task_struct *p, int cpu)
        {
                struct rq *rq = cpu_rq(cpu);
                ......
                if (... && !cpus_share_cache(smp_processor_id(), cpu)) {
                        ......
                        ttwu_queue_remote(p, cpu); /* will trigger RES IPI */
                        return;
                }
                ......
                ttwu_do_activate(rq, p, 0); /* access target's rq directly */
                ......
        }

In real hardware, the cpus on the same socket share L3 cache, so one won't
trigger a resched IPIs when wakeup a task on others. But QEMU doesn't present a
virtual L3 cache info for VM, then the linux guest will trigger lots of RES IPIs
under some workloads even if the virtual cpus belongs to the same virtual 
socket.

For KVM, there will be lots of vmexit due to guest send IPIs.
The workload is a SAP HANA's testsuite, we run it one round(about 40 minuates)
and observe the (Suse11sp3)Guest's amounts of RES IPIs which triggering during
the period:
        No-L3           With-L3(applied this patch)
cpu0:   363890          44582
cpu1:   373405          43109
cpu2:   340783          43797
cpu3:   333854          43409
cpu4:   327170          40038
cpu5:   325491          39922
cpu6:   319129          42391
cpu7:   306480          41035
cpu8:   161139          32188
cpu9:   164649          31024
cpu10:  149823          30398
cpu11:  149823          32455
cpu12:  164830          35143
cpu13:  172269          35805
cpu14:  179979          33898
cpu15:  194505          32754
avg:    268963.6        40129.8

The VM's topology is "1*socket 8*cores 2*threads".
After present virtual L3 cache info for VM, the amounts of RES IPIs in guest
reduce 85%.

For KVM, vcpus send IPIs will cause vmexit which is expensive, so it can cause
severe performance degradation. We had tested the overall system performance if
vcpus actually run on sparate physical socket. With L3 cache, the performance
improves 7.2%~33.1%(avg:15.7%).

Signed-off-by: Longpeng(Mike) <address@hidden>
Reviewed-by: Michael S. Tsirkin <address@hidden>
Signed-off-by: Michael S. Tsirkin <address@hidden>
---
 include/hw/i386/pc.h |  9 +++++++++
 target-i386/cpu.h    |  6 ++++++
 target-i386/cpu.c    | 49 ++++++++++++++++++++++++++++++++++++++++++++-----
 3 files changed, 59 insertions(+), 5 deletions(-)

diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
index 8ad6f15..ebba151 100644
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -368,7 +368,16 @@ int e820_add_entry(uint64_t, uint64_t, uint32_t);
 int e820_get_num_entries(void);
 bool e820_get_entry(int, uint32_t, uint64_t *, uint64_t *);
 
+#define PC_COMPAT_2_8 \
+    {\
+        .driver   = TYPE_X86_CPU,\
+        .property = "l3-cache",\
+        .value    = "off",\
+    },
+
+
 #define PC_COMPAT_2_7 \
+    PC_COMPAT_2_8 \
     HW_COMPAT_2_7
 
 #define PC_COMPAT_2_6 \
diff --git a/target-i386/cpu.h b/target-i386/cpu.h
index cf14bcb..bb3ffda 100644
--- a/target-i386/cpu.h
+++ b/target-i386/cpu.h
@@ -1207,6 +1207,12 @@ struct X86CPU {
      */
     bool enable_lmce;
 
+    /* Compatibility bits for old machine types.
+     * If true present virtual l3 cache for VM, the vcpus in the same virtual
+     * socket share an virtual l3 cache.
+     */
+    bool enable_l3_cache;
+
     /* Compatibility bits for old machine types: */
     bool enable_cpuid_0xb;
 
diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index ec674dc..5a5299a 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -57,6 +57,7 @@
 #define CPUID_2_L1D_32KB_8WAY_64B 0x2c
 #define CPUID_2_L1I_32KB_8WAY_64B 0x30
 #define CPUID_2_L2_2MB_8WAY_64B   0x7d
+#define CPUID_2_L3_16MB_16WAY_64B 0x4d
 
 
 /* CPUID Leaf 4 constants: */
@@ -131,11 +132,18 @@
 #define L2_LINES_PER_TAG       1
 #define L2_SIZE_KB_AMD       512
 
-/* No L3 cache: */
+/* Level 3 unified cache: */
 #define L3_SIZE_KB             0 /* disabled */
 #define L3_ASSOCIATIVITY       0 /* disabled */
 #define L3_LINES_PER_TAG       0 /* disabled */
 #define L3_LINE_SIZE           0 /* disabled */
+#define L3_N_LINE_SIZE         64
+#define L3_N_ASSOCIATIVITY     16
+#define L3_N_SETS           16384
+#define L3_N_PARTITIONS         1
+#define L3_N_DESCRIPTOR CPUID_2_L3_16MB_16WAY_64B
+#define L3_N_LINES_PER_TAG      1
+#define L3_N_SIZE_KB_AMD    16384
 
 /* TLB definitions: */
 
@@ -2279,6 +2287,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
uint32_t count,
 {
     X86CPU *cpu = x86_env_get_cpu(env);
     CPUState *cs = CPU(cpu);
+    uint32_t pkg_offset;
 
     /* test if maximum index reached */
     if (index & 0x80000000) {
@@ -2332,7 +2341,11 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
uint32_t count,
         }
         *eax = 1; /* Number of CPUID[EAX=2] calls required */
         *ebx = 0;
-        *ecx = 0;
+        if (!cpu->enable_l3_cache) {
+            *ecx = 0;
+        } else {
+            *ecx = L3_N_DESCRIPTOR;
+        }
         *edx = (L1D_DESCRIPTOR << 16) | \
                (L1I_DESCRIPTOR <<  8) | \
                (L2_DESCRIPTOR);
@@ -2378,6 +2391,25 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
uint32_t count,
                 *ecx = L2_SETS - 1;
                 *edx = CPUID_4_NO_INVD_SHARING;
                 break;
+            case 3: /* L3 cache info */
+                if (!cpu->enable_l3_cache) {
+                    *eax = 0;
+                    *ebx = 0;
+                    *ecx = 0;
+                    *edx = 0;
+                    break;
+                }
+                *eax |= CPUID_4_TYPE_UNIFIED | \
+                        CPUID_4_LEVEL(3) | \
+                        CPUID_4_SELF_INIT_LEVEL;
+                pkg_offset = apicid_pkg_offset(cs->nr_cores, cs->nr_threads);
+                *eax |= ((1 << pkg_offset) - 1) << 14;
+                *ebx = (L3_N_LINE_SIZE - 1) | \
+                       ((L3_N_PARTITIONS - 1) << 12) | \
+                       ((L3_N_ASSOCIATIVITY - 1) << 22);
+                *ecx = L3_N_SETS - 1;
+                *edx = CPUID_4_INCLUSIVE | CPUID_4_COMPLEX_IDX;
+                break;
             default: /* end of info */
                 *eax = 0;
                 *ebx = 0;
@@ -2589,9 +2621,15 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
uint32_t count,
         *ecx = (L2_SIZE_KB_AMD << 16) | \
                (AMD_ENC_ASSOC(L2_ASSOCIATIVITY) << 12) | \
                (L2_LINES_PER_TAG << 8) | (L2_LINE_SIZE);
-        *edx = ((L3_SIZE_KB/512) << 18) | \
-               (AMD_ENC_ASSOC(L3_ASSOCIATIVITY) << 12) | \
-               (L3_LINES_PER_TAG << 8) | (L3_LINE_SIZE);
+        if (!cpu->enable_l3_cache) {
+            *edx = ((L3_SIZE_KB / 512) << 18) | \
+                   (AMD_ENC_ASSOC(L3_ASSOCIATIVITY) << 12) | \
+                   (L3_LINES_PER_TAG << 8) | (L3_LINE_SIZE);
+        } else {
+            *edx = ((L3_N_SIZE_KB_AMD / 512) << 18) | \
+                   (AMD_ENC_ASSOC(L3_N_ASSOCIATIVITY) << 12) | \
+                   (L3_N_LINES_PER_TAG << 8) | (L3_N_LINE_SIZE);
+        }
         break;
     case 0x80000007:
         *eax = 0;
@@ -3368,6 +3406,7 @@ static Property x86_cpu_properties[] = {
     DEFINE_PROP_STRING("hv-vendor-id", X86CPU, hyperv_vendor_id),
     DEFINE_PROP_BOOL("cpuid-0xb", X86CPU, enable_cpuid_0xb, true),
     DEFINE_PROP_BOOL("lmce", X86CPU, enable_lmce, false),
+    DEFINE_PROP_BOOL("l3-cache", X86CPU, enable_l3_cache, true),
     DEFINE_PROP_END_OF_LIST()
 };
 
-- 
MST