From: Zhao Liu <zhao1.liu@xxxxxxxxx>
For i-cache and d-cache, current QEMU hardcodes the maximum IDs for CPUs
sharing cache (CPUID.04H.00H:EAX[bits 25:14] and CPUID.04H.01H:EAX[bits
25:14]) to 0, and this means i-cache and d-cache are shared in the SMT
level.
This is correct if there's single thread per core, but is wrong for the
hyper threading case (one core contains multiple threads) since the
i-cache and d-cache are shared in the core level other than SMT level.
For AMD CPU, commit 8f4202fb1080 ("i386: Populate AMD Processor Cache
Information for cpuid 0x8000001D") has already introduced i/d cache
topology as core level by default.
Therefore, in order to be compatible with both multi-threaded and
single-threaded situations, we should set i-cache and d-cache be shared
at the core level by default.
This fix changes the default i/d cache topology from per-thread to
per-core. Potentially, this change in L1 cache topology may affect the
performance of the VM if the user does not specifically specify the
topology or bind the vCPU. However, the way to achieve optimal
performance should be to create a reasonable topology and set the
appropriate vCPU affinity without relying on QEMU's default topology
structure.
Fixes: 7e3482f82480 ("i386: Helpers to encode cache information consistently")
Suggested-by: Robert Hoo <robert.hu@xxxxxxxxxxxxxxx>
Signed-off-by: Zhao Liu <zhao1.liu@xxxxxxxxx>
Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
Tested-by: Babu Moger <babu.moger@xxxxxxx>
Tested-by: Yongwei Ma <yongwei.ma@xxxxxxxxx>
Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
---
Changes since v3:
* Change the description of current i/d cache encoding status to avoid
misleading to "architectural rules". (Xiaoyao)
Changes since v1:
* Split this fix from the patch named "i386/cpu: Fix number of
addressable IDs in CPUID.04H".
* Add the explanation of the impact on performance. (Xiaoyao)
---
target/i386/cpu.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 95d5f16cd5eb..5a3678a789cf 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -6113,12 +6113,12 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
switch (count) {
case 0: /* L1 dcache info */
encode_cache_cpuid4(env->cache_info_cpuid4.l1d_cache,
- 1, cs->nr_cores,
+ cs->nr_threads, cs->nr_cores,
eax, ebx, ecx, edx);
break;
case 1: /* L1 icache info */
encode_cache_cpuid4(env->cache_info_cpuid4.l1i_cache,
- 1, cs->nr_cores,
+ cs->nr_threads, cs->nr_cores,
eax, ebx, ecx, edx);
break;
case 2: /* L2 cache info */
--
2.34.1
