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

[Peter Maydell]

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.
>
> Can someone confirm/deny this?  If so, it's a breaking change that
> perhaps could be mentioned in the Changelog to save others the time
> when they upgrade. Affected educational OS include MIT's xv6 and
> Stanford's pintos OS.
>
> Thanks for all the work you do on qemu!
>
>  - Godmar
>
> [1] 
> https://github.com/mit-pdos/xv6-public/blob/eeb7b415dbcb12cc362d0783e41c3d1f44066b17/mp.c
> [2] 
> https://qemu-project.gitlab.io/qemu/about/removed-features.html#smp-invalid-topologies-removed-in-5-2
>
> (I'm typing this email in gmail using the plaintext setting, hopefully
> it'll preserve line breaks.)

thanks
-- PMM