[Qemu-devel] CPU topology and hyperthreading

[Mohammed Gamal]

Hi All,

I have a question regarding the way CPU topology is exposed to the guest.

On a 4-core Amazon AWS VM I can see the CPU topology exposed to the
guest in the following manner:

# lstopo
Machine (7480MB)
  Socket L#0 + L3 L#0 (25MB)
    L2 L#0 (256KB) + L1d L#0 (32KB) + L1i L#0 (32KB) + Core L#0
      PU L#0 (P#0)
      PU L#1 (P#2)
    L2 L#1 (256KB) + L1d L#1 (32KB) + L1i L#1 (32KB) + Core L#1
      PU L#2 (P#1)
      PU L#3 (P#3)
[...]

Now trying to emulate this topology in qemu/kvm using the following
command line options:
-cpu Haswell,+ht -smp 4,sockets=1,cores=2,maxcpus=64,threads=2

as well as
-cpu kvm64,+ht -smp 4,sockets=1,cores=2,maxcpus=64,threads=2


Shows me something like this:

# lstopo
Machine (1870MB)
  Socket L#0
     L2 L#0 (4096KB) + Core L#0
       L1d L#0 (32KB) + L1i L#0 (32KB) + PU L#0 (P#0)
       L1d L#1 (32KB) + L1i L#1 (32KB) + PU L#1 (P#1)
     L2 L#1 (4096KB) + Core L#1
       L1d L#2 (32KB) + L1i L#2 (32KB) + PU L#2 (P#2)
       L1d L#3 (32KB) + L1i L#3 (32KB) + PU L#3 (P#3)
[...]

In other words, qemu exposes each hyperthread as if it has its own L1
data and instruction caches. Should the be a correct behavior?

In all cases, what gets exposed in the guests's /proc/cpuinfo would be
the same, but I wonder why the topology is different?

Regards,
Mohammed