Re: [Qemu-devel] [PULL 11/13] target-i386: forward CPUID cache leaves when -cpu host is used

[Peter Lieven <pl@kamp.de>]

On 19.11.2013 11:47, Paolo Bonzini wrote:
> Il 19/11/2013 11:25, Peter Lieven ha scritto:
>> ~/git/qemu$ x86_64-softmmu/qemu-system-x86_64 -m 2048 -drive
>> if=virtio,file=iscsi://172.21.200.45/iqn.2001-05.com.equallogic:0-8a0906-9d95c510a-344001d54795289f-2012-r2-1-7-0/0,format=raw,cache=writeback,aio=native
>> -smp 2,cores=2,threads=1,sockets=1 -cpu host -monitor stdio -vnc :1
>> -enable-kvm -usb -usbdevice tablet -vga cirrus -global
>> virtio-blk-pci.scsi=off  -serial null  -parallel null -boot c
> What is your host CPU's topology
I tested it with 1 socket, 2 cores, 2 threads per core (my workstation) and 2 sockets, 8 cores per socket, 2 threads per thread.
Both crash.
>
>> With just -smp 2 it works. However, have a look at my other email I
>> think there is a bug in smp_parse, because -smp 2 yields
>> cpus=2,cores=1,threads=1,sockets=1 whereas I think cores should
>> be 2.
> The code matching the comment in vl.c ("compute missing values, prefer
> sockets over cores over threads") would be like "-smp
> cpu=2,cores=1,threads=1,sockets=2", giving this code:
>
>          if (cpus == 0) {
>              sockets = sockets > 0 ? sockets : 1;
>              cores = cores > 0 ? cores : 1;
>              threads = threads > 0 ? threads : 1;
>              cpus = cores * threads * sockets;
>          } else if (sockets == 0) {
>              cores = cores > 0 ? cores : 1;
>              threads = threads > 0 ? threads : 1;
>              sockets = cpus / (cores * threads);
>          } else if (cores == 0) {
>              threads = threads > 0 ? threads : 1;
>              cores = cpus / (sockets * threads);
>          } else {
>              threads = cpus / (sockets * cores);
>          }
I am fine with either of the both variants, it should just
be consistent ;-)
>
> What you suggest is cores over threads over sockets:
>
>          if (cpus == 0) {
>              cores = cores > 0 ? cores : 1;
>              threads = threads > 0 ? threads : 1;
>              sockets = sockets > 0 ? sockets : 1;
>              cpus = cores * threads * sockets;
>          } else if (cores == 0) {
>              threads = threads > 0 ? threads : 1;
>              sockets = sockets > 0 ? sockets : 1;
>              cores = cpus / (threads * sockets);
>          } else if (threads == 0) {
>              sockets = sockets > 0 ? sockets : 1;
>              threads = cpus / (cores * sockets);
>          } else {
>              sockets = cpus / (cores * threads);
>          }
>
> Can you test which of these two work?  But I agree it's best to disable
> cache-leaf forwarding.
The problem is, its broken because at least cpuid index 4 includes a hint
to the number of cores and threads. I think we have to disable the cache
leaf forwarding if the qemu cpu topology does not match the host topology.

I also tried to fix index 4, but this alone seems to be not enough. at least
in index 2 seems also to be some info about cores and threads (which is currently
not there).