On Thu, 20 Jan 2022 09:46:09 -0500
Godmar Back <god...@gmail.com> wrote:
> [this is a follow-up email; I took qemu-discuss and qemu-devel off the
> distribution list]
>
> On Thu, Jan 20, 2022 at 4:04 AM Igor Mammedov <imamm...@redhat.com> wrote:
> >
> > Legacy MP table is not actively maintained part of the code,
> > hence it's configuration which is not tested.
> > However if someone is interested in maintaining this, one
> > should contribute at least a testcase that will warn developers
> > early if usecase is broken. We can't promise not breaking it
> > ever but at least we would be able to document any breaking
> > changes in release notes.
> >
>
> May I ask a clarification question? Is it correct that the only
> option to not use the MP tables and learn the CPU configuration would
> be to implement support for ACPI? For a small educational OS like xv6
> or Pintos, we're aiming at keeping the codebase small, which is why so
> far we haven't taken this step. Is there a way to learn the CPU
> configuration without using ACPI that involves relatively little code?
I do see value from education point of view in legacy MP table,
however I'd guess its support will continue to deteriorate over
the time, as ACPI is what majority of mainstream operating systems
currently use.
If you wish to use qemu-6.2 and later and MP tables you have 2 options
to do this:
1) use 6.1 machine type (or any older one):
qemu-system-i386 -M pc-i440fx-6.1 -smp 4 ...
2) use default (latest) machine type and define explicit cpu layout
supported by SeaBIOS
qemu-system-i386 -smp 4,sockets=4 ...
As for adding ACPI support to xv6, is not so difficult if you only use
it for detecting CPUs. You only need to locate RSDP which points to
a list of ACPI tables ((X)RSDT) and then jump and parse MADT table
which is analog of MP table. It's all very well documented in
ACPI spec and you can use SeaBIOS code for inspiration as well,
look for RSDP_SIGNATURE as starting point (SeaBIOS does a lot more
stuff with ACPI but you don't really need it (you can start with
minimum of necessary code and then build up on top as needed)).
Alternative approach to discover present CPUs with ad-hock approach
SeaBIOS use(s|d) to build its own MADT tables (note: since QEMU-1.7
it uses ACPI tables provided by QEMU).
See smp_scan() + handle_smp() for an example.
> - Godmar
>
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