Re: MP tables do not report multiple CPUs in Qemu 6.2.0 on x86 when given -smp cpus=n flag

[Ani Sinha]

On Wed, 19 Jan 2022, Peter Maydell wrote:

> On Wed, 19 Jan 2022 at 14:44, Godmar Back <gback@cs.vt.edu> wrote:
> > after upgrading to 6.2.0, I observed that code such as MIT's xv6 (see
> > [1]) is no longer able to detect multiple CPUs.  Their code works in
> > 6.1.1, however.
>
> Hi; this isn't a great place for reporting QEMU bugs, because
> it's more of a user-to-user discussion list. Not all QEMU
> developers read it. I've cc'd the ACPI maintainers, who
> hopefully may have an idea about what's happening here.
> You could also file a bug at
> https://gitlab.com/qemu-project/qemu/-/issues
>
> > I built 6.1.1 from source and 6.2.0 from source and I have also tested
> > with CentOS stream's 6.1.1 qemu-kvm and was able to pinpoint this
> > change to these 2 versions of qemu. I am using qemu-system-i386
> > specifically.
> >
> > I tried to go through the ChangeLog to see if the `-smp` option was
> > deprecated or changed.  I found this note [2] about invalid topologies
> > having been removed in 5.2. Here's what I found after long
> > experimentation:
> >
> > The legacy MP tables appear to work only if you specify the longform
> > `-smp cpus=4,cores=1,threads=1,sockets=4` in 6.2.0.  If you specify
> > `-smp 4` or `-smp cpus=4` it will not work in 6.2.0 (it worked in
> > 6.1.1). I am guessing that perhaps the MP tables add entries for each
> > socket, but that perhaps the behavior of the shortcuts `-smp n` and
> > `-smp cpus=n` was changed to influence the number of cores rather than
> > sockets.
> >
> > In other words, `-smp cpus=n` now means `-smp
> > cpus=n,cores=n,threads=1,sockets=1` whereas in 6.1.1 and before it
> > meant `-smp cpus=n,cores=1,threads=1,sockets=n`. I note that
> > specifying `-smp cpus=4,cores=4,threads=1,sockets=1` in 6.1.1 also
> > does not create 4 entries in the legacy MP tables.
> >

My suspicion is that the following commit might have introduced a
regression:

commit 86ce2d28fa09d15547b5cabdc264cadfb53a848c
Author: Yanan Wang <wangyanan55@huawei.com>
Date:   Tue Oct 26 11:46:58 2021 +0800

    hw/core/machine: Split out the smp parsing code

    We are going to introduce an unit test for the parser smp_parse()
    in hw/core/machine.c, but now machine.c is only built in softmmu.

    In order to solve the build dependency on the smp parsing code and
    avoid building unrelated stuff for the unit tests, move the tested
    code from machine.c into a separate file, i.e., machine-smp.c and
    build it in common field.

    Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
    Reviewed-by: Andrew Jones <drjones@redhat.com>
    Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
    Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
    Message-Id: <20211026034659.22040-2-wangyanan55@huawei.com>
    Acked-by: Eduardo Habkost <ehabkost@redhat.com>
    Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

I am still checking.