[Bug 1856335] Re: Cache Layout wrong on many Zen Arch CPUs

[Jan Klos]

The problem is caused by the fact that with Ryzen CPUs with disabled
cores, the APIC IDs are not sequential on host - in order for cache
topology to be configured properly, there is a 'hole' in APIC ID and
core ID numbering (I have added full output of cpuid for my 3900X).
Unfortunately, adding holes to the numbering is the only way to achieve
what is needed for 3 cores per CCX as CPUID Fn8000_001D_EAX
NumSharingCache parameter rounds to  powers of two (for Ryzen 3100 with
2 cores per CCX, lowering NumSharingCache should also work, correctly
setting the L3 cache cores with their IDs still being sequential).

A small hack in x86_apicid_from_topo_ids() in include/hw/i386/topology.h
can introduce a correct numbering (at least if you do not have epyc set
as your cpu, then _epyc variant of the functions are used). But to fix
this properly will probably require some thought - maybe introduce the
ability to assign APIC IDs directly somehow? Or the ability to specify
the 'holes' somehow in the -smt param, or maybe -cpu host,topoext=on
should do this automatically? I don't know...

e.g. For 3 core per CCX CPUs, to fix this, at
include/hw/i386/topology.h:220 change:

(topo_ids->core_id << apicid_core_offset(topo_info)) |

to

((topo_ids->core_id + (topo_ids->core_id / 3)) <<
apicid_core_offset(topo_info)) |


The cache topology is now correct (-cpu 
host,topoext=on,hv-time,hv-relaxed,hv-vapic,hv-spinlocks=0x1fff,host-cache-info=on
 -smp 18,sockets=1,dies=1,cores=9,threads=2), even in Windows:

Logical Processor to Cache Map:
**----------------  Data Cache          0, Level 1,   32 KB, Assoc   8, 
LineSize  64
**----------------  Instruction Cache   0, Level 1,   32 KB, Assoc   8, 
LineSize  64
**----------------  Unified Cache       0, Level 2,  512 KB, Assoc   8, 
LineSize  64
******------------  Unified Cache       1, Level 3,   16 MB, Assoc  16, 
LineSize  64
--**--------------  Data Cache          1, Level 1,   32 KB, Assoc   8, 
LineSize  64
--**--------------  Instruction Cache   1, Level 1,   32 KB, Assoc   8, 
LineSize  64
--**--------------  Unified Cache       2, Level 2,  512 KB, Assoc   8, 
LineSize  64
----**------------  Data Cache          2, Level 1,   32 KB, Assoc   8, 
LineSize  64
----**------------  Instruction Cache   2, Level 1,   32 KB, Assoc   8, 
LineSize  64
----**------------  Unified Cache       3, Level 2,  512 KB, Assoc   8, 
LineSize  64
------**----------  Data Cache          3, Level 1,   32 KB, Assoc   8, 
LineSize  64
------**----------  Instruction Cache   3, Level 1,   32 KB, Assoc   8, 
LineSize  64
------**----------  Unified Cache       4, Level 2,  512 KB, Assoc   8, 
LineSize  64
------******------  Unified Cache       5, Level 3,   16 MB, Assoc  16, 
LineSize  64
--------**--------  Data Cache          4, Level 1,   32 KB, Assoc   8, 
LineSize  64
--------**--------  Instruction Cache   4, Level 1,   32 KB, Assoc   8, 
LineSize  64
--------**--------  Unified Cache       6, Level 2,  512 KB, Assoc   8, 
LineSize  64
----------**------  Data Cache          5, Level 1,   32 KB, Assoc   8, 
LineSize  64
----------**------  Instruction Cache   5, Level 1,   32 KB, Assoc   8, 
LineSize  64
----------**------  Unified Cache       7, Level 2,  512 KB, Assoc   8, 
LineSize  64
------------**----  Data Cache          6, Level 1,   32 KB, Assoc   8, 
LineSize  64
------------**----  Instruction Cache   6, Level 1,   32 KB, Assoc   8, 
LineSize  64
------------**----  Unified Cache       8, Level 2,  512 KB, Assoc   8, 
LineSize  64
------------******  Unified Cache       9, Level 3,   16 MB, Assoc  16, 
LineSize  64


** Attachment added: "cpuid.txt"
   
https://bugs.launchpad.net/qemu/+bug/1856335/+attachment/5383184/+files/cpuid.txt

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https://bugs.launchpad.net/bugs/1856335

Title:
  Cache Layout wrong on many Zen Arch CPUs

Status in QEMU:
  New

Bug description:
  AMD CPUs have L3 cache per 2, 3 or 4 cores. Currently, TOPOEXT seems
  to always map Cache ass if it was an 4-Core per CCX CPU, which is
  incorrect, and costs upwards 30% performance (more realistically 10%)
  in L3 Cache Layout aware applications.

  Example on a 4-CCX CPU (1950X /w 8 Cores and no SMT):

    <cpu mode='custom' match='exact' check='full'>
      <model fallback='forbid'>EPYC-IBPB</model>
      <vendor>AMD</vendor>
      <topology sockets='1' cores='8' threads='1'/>

  In windows, coreinfo reports correctly:

  ****----  Unified Cache 1, Level 3,    8 MB, Assoc  16, LineSize  64
  ----****  Unified Cache 6, Level 3,    8 MB, Assoc  16, LineSize  64

  On a 3-CCX CPU (3960X /w 6 cores and no SMT):

   <cpu mode='custom' match='exact' check='full'>
      <model fallback='forbid'>EPYC-IBPB</model>
      <vendor>AMD</vendor>
      <topology sockets='1' cores='6' threads='1'/>

  in windows, coreinfo reports incorrectly:

  ****--  Unified Cache  1, Level 3,    8 MB, Assoc  16, LineSize  64
  ----**  Unified Cache  6, Level 3,    8 MB, Assoc  16, LineSize  64

  Validated against 3.0, 3.1, 4.1 and 4.2 versions of qemu-kvm.

  With newer Qemu there is a fix (that does behave correctly) in using the dies 
parameter:
   <qemu:arg value='cores=3,threads=1,dies=2,sockets=1'/>

  The problem is that the dies are exposed differently than how AMD does
  it natively, they are exposed to Windows as sockets, which means, that
  if you are nto a business user, you can't ever have a machine with
  more than two CCX (6 cores) as consumer versions of Windows only
  supports two sockets. (Should this be reported as a separate bug?)

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