Re: [PATCH 0/2] i386: fixup number of logical CPUs when host-cache-info=on

[Moger, Babu]

On 5/24/22 18:23, Alejandro Jimenez wrote:
> On 5/24/2022 3:48 PM, Moger, Babu wrote:
>>
>> On 5/24/22 10:19, Igor Mammedov wrote:
>>> On Tue, 24 May 2022 11:10:18 -0400
>>> Igor Mammedov <imammedo@redhat.com> wrote:
>>>
>>> CCing AMD folks as that might be of interest to them
>>
>> I am trying to recreate the bug on my AMD system here.. Seeing this
>> message..
>>
>> qemu-system-x86_64: -numa node,nodeid=0,memdev=ram-node0: memdev=ram-node0
>> is ambiguous
>>
>> Here is my command line..
>>
>> #qemu-system-x86_64 -name rhel8 -m 4096 -hda vdisk.qcow2 -enable-kvm -net
>> nic  -nographic -machine q35,accel=kvm -cpu
>> host,host-cache-info=on,l3-cache=off -smp
>> 20,sockets=2,dies=1,cores=10,threads=1 -numa
>> node,nodeid=0,memdev=ram-node0 -numa node,nodeid=1,memdev=ram-node1 -numa
>> cpu,socket-id=0,node-id=0 -numa cpu,socket-id=1,node-id=1
>>
>> Am I missing something?
> Hi Babu,
>
> Hopefully this will help you reproduce the issue if you are testing on
> Milan/Genoa. Joao (CC'd) pointed out this warning to me late last year,
> while I was working on patches for encoding the topology CPUID leaf in
> different Zen platforms.
>
> What I found from my experiments on Milan, is that the warning will
> appear whenever the NUMA topology requested in QEMU cmdline assigns a
> number of CPUs to each node that is smaller than the default # of CPUs
> sharing a LLC on the host platform. In short, on a Milan host where we
> have 16 CPUs sharing a CCX:

Yes. I recreated the issue with this following command line.

#qemu-system-x86_64 -name rhel8 -m 4096 -hda vdisk.qcow2 -enable-kvm -net
nic  -nographic -machine q35,accel=kvm -cpu host,+topoext -smp
16,sockets=1,dies=1,cores=16,threads=1 -object
memory-backend-ram,id=ram-node0,size=2G -object
memory-backend-ram,id=ram-node1,size=2G  -numa
node,nodeid=0,cpus=0-7,memdev=ram-node0 -numa
node,nodeid=1,cpus=8-15,memdev=ram-node1

But solving this will be bit complicated. For AMD, this information comes
from CPUID 0x8000001d. But, when this cpuid is being populated we don't
have all the information about numa nodes etc..

But you can work-around it by modifying the command line by including
dies(dies=2 in this case) information.  Something like this.

#qemu-system-x86_64 -name rhel8 -m 4096 -hda vdisk.qcow2 -enable-kvm -net
nic  -nographic -machine q35,accel=kvm -cpu
host,+topoext,host-cache-info=on -smp
16,sockets=1,dies=2,cores=8,threads=1 -object
memory-backend-ram,id=ram-node0,size=2G -object
memory-backend-ram,id=ram-node1,size=2G  -numa
node,nodeid=0,cpus=0-7,memdev=ram-node0 -numa
node,nodeid=1,cpus=8-15,memdev=ram-node1

But this may not be acceptable solution in all the cases.

>
> # cat /sys/devices/system/cpu/cpu0/cache/index3/shared_cpu_list
> 0-7,128-135
>
> If a guest is launched with the following arguments:
>
> -cpu host,+topoext \
> -smp cpus=64,cores=32,threads=2,sockets=1 \
> -numa node,nodeid=0,cpus=0-7 -numa node,nodeid=1,cpus=8-15 \
> -numa node,nodeid=2,cpus=16-23 -numa node,nodeid=3,cpus=24-31 \
> -numa node,nodeid=4,cpus=32-39 -numa node,nodeid=5,cpus=40-47 \
> -numa node,nodeid=6,cpus=48-55 -numa node,nodeid=7,cpus=56-63 \
>
> it assigns 8 cpus to each NUMA node, causing the error above to be
> displayed.
>
> Note that ultimately the guest topology is built based on the NUMA
> information, so the LLC domains on the guest only end up spanning a
> single NUMA node. e.g.:
>
> # cat /sys/devices/system/cpu/cpu0/cache/index3/shared_cpu_list
> 0-7
>
> Hope that helps,
> Alejandro
>>
>>
>>>
>>>> Igor Mammedov (2):
>>>>    x86: cpu: make sure number of addressable IDs for processor cores
>>>>      meets the spec
>>>>    x86: cpu: fixup number of addressable IDs for logical processors
>>>>      sharing cache
>>>>
>>>>   target/i386/cpu.c | 20 ++++++++++++++++----
>>>>   1 file changed, 16 insertions(+), 4 deletions(-)
>>>>
>
-- 
Thanks
Babu Moger