On 14/9/23 09:21, Zhao Liu wrote:
From: Zhuocheng Ding <zhuocheng.ding@xxxxxxxxx>
smp command has the "clusters" parameter but x86 hasn't supported that
level. "cluster" is a CPU topology level concept above cores, in which
the cores may share some resources (L2 cache or some others like L3
cache tags, depending on the Archs) [1][2]. For x86, the resource shared
by cores at the cluster level is mainly the L2 cache.
However, using cluster to define x86's L2 cache topology will cause the
compatibility problem:
Currently, x86 defaults that the L2 cache is shared in one core, which
actually implies a default setting "cores per L2 cache is 1" and
therefore implicitly defaults to having as many L2 caches as cores.
For example (i386 PC machine):
-smp 16,sockets=2,dies=2,cores=2,threads=2,maxcpus=16 (*)
Considering the topology of the L2 cache, this (*) implicitly means "1
core per L2 cache" and "2 L2 caches per die".
If we use cluster to configure L2 cache topology with the new default
setting "clusters per L2 cache is 1", the above semantics will change
to "2 cores per cluster" and "1 cluster per L2 cache", that is, "2
cores per L2 cache".
So the same command (*) will cause changes in the L2 cache topology,
further affecting the performance of the virtual machine.
Therefore, x86 should only treat cluster as a cpu topology level and
avoid using it to change L2 cache by default for compatibility.
"cluster" in smp is the CPU topology level which is between "core" and
die.
For x86, the "cluster" in smp is corresponding to the module level [2],
which is above the core level. So use the "module" other than "cluster"
in i386 code.
And please note that x86 already has a cpu topology level also named
"cluster" [3], this level is at the upper level of the package. Here,
the cluster in x86 cpu topology is completely different from the
"clusters" as the smp parameter. After the module level is introduced,
the cluster as the smp parameter will actually refer to the module level
of x86.
[1]: 864c3b5c32f0 ("hw/core/machine: Introduce CPU cluster topology support")
[2]: Yanan's comment about "cluster",
https://lists.gnu.org/archive/html/qemu-devel/2023-02/msg04051.html
[3]: SDM, vol.3, ch.9, 9.9.1 Hierarchical Mapping of Shared Resources.
Signed-off-by: Zhuocheng Ding <zhuocheng.ding@xxxxxxxxx>
Co-developed-by: Zhao Liu <zhao1.liu@xxxxxxxxx>
Signed-off-by: Zhao Liu <zhao1.liu@xxxxxxxxx>
Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
---
Changes since v1:
* The background of the introduction of the "cluster" parameter and its
exact meaning were revised according to Yanan's explanation. (Yanan)
---
hw/i386/x86.c | 1 +
target/i386/cpu.c | 1 +
target/i386/cpu.h | 5 +++++
3 files changed, 7 insertions(+)
diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index 470257b92240..556e80f29764 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -1903,6 +1903,11 @@ typedef struct CPUArchState {
/* Number of dies within this CPU package. */
unsigned nr_dies;
+ /*
+ * Number of modules within this CPU package.
+ * Module level in x86 cpu topology is corresponding to smp.clusters.
+ */
+ unsigned nr_modules;
} CPUX86State;
It would be really useful to have an ASCII art comment showing
the architecture topology. Also for clarity the topo fields from
CPU[Arch]State could be moved into a 'topo' sub structure, or even
clearer would be to re-use the X86CPUTopoIDs structure?
