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Re: x86_64 emulation on ppc64le POWER9 hardware - no multi threading

From: Cédric Le Goater
Subject: Re: x86_64 emulation on ppc64le POWER9 hardware - no multi threading
Date: Mon, 3 Aug 2020 19:37:32 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

Again, with a correct answer,

On 8/3/20 3:47 PM, Kim-Norman Sahm wrote:
> hi,
> i’m running Debian 10 on POWER9 and would like to spawn x86_64 emulated VMs.
> The virtual machine is configured to run with 8 vpus but it’s very slow.
> On the host you can see that the qemu-system-x86_64 process is using just one 
> core!
> Ppc64le guests are using multi cores, so its looks like an config problem or 
> software bug with the x86 emulator.

On a Power9 host (20.04), I did the following changes to an x86 domain :

  <domain type='qemu' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>

and at the end of the domain file  :
    <qemu:arg value='-accel'/>
    <qemu:arg value='tcg,thread=multi'/>

The topology is defined as : 

  <cpu mode='custom' match='exact' check='none'>
    <model fallback='forbid'>qemu64</model>
    <topology sockets='1' cores='12' threads='2'/>

The VM is using MTTCG, as reported by top on the host :

    PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
5761 libvirt+  20   0   23.5g   1.8g  21888 R  2167   1.4   6:10.82 

In the guest :

$ lscpu 
Architecture:                    x86_64
CPU op-mode(s):                  32-bit, 64-bit
Byte Order:                      Little Endian
Address sizes:                   40 bits physical, 48 bits virtual
CPU(s):                          24
On-line CPU(s) list:             0-23
Thread(s) per core:              1
Core(s) per socket:              24
Socket(s):                       1
NUMA node(s):                    1
Vendor ID:                       AuthenticAMD
CPU family:                      6
Model:                           6
Model name:                      QEMU Virtual CPU version 2.1.0
Stepping:                        3
CPU MHz:                         511.035
BogoMIPS:                        1022.07
Virtualization:                  AMD-V
L1d cache:                       1.5 MiB
L1i cache:                       1.5 MiB
L2 cache:                        12 MiB
NUMA node0 CPU(s):               0-23
Vulnerability L1tf:              Not affected
Vulnerability Mds:               Not affected
Vulnerability Meltdown:          Not affected
Vulnerability Spec store bypass: Vulnerable
Vulnerability Spectre v1:        Mitigation; usercopy/swapgs barriers and __user
                                  pointer sanitization
Vulnerability Spectre v2:        Mitigation; Full generic retpoline, STIBP disab
                                 led, RSB filling
Flags:                           fpu de pse tsc msr pae mce cx8 apic sep mtrr pg
                                 e mca cmov pat pse36 clflush mmx fxsr sse sse2 
                                 ht syscall nx lm nopl cpuid pni cx16 popcnt hyp
                                 ervisor lahf_lm cmp_legacy svm abm sse4a 3dnowp
                                 refetch vmmcall



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