Re: [RFC PATCH v2 5/6] hw/arm/virt-acpi-build: Add PPTT table

[Andrew Jones]

On Tue, May 18, 2021 at 09:05:39PM +0200, Andrew Jones wrote:
> The problem is that -smp 4,maxcpus=8 doesn't error out today, even though
> it doesn't do anything. OTOH, -smp 4,cores=2 doesn't error out either, but
> we're proposing that it should. Maybe we can start erroring out when
> cpus != maxcpus until hot plug is supported?
>

The more I think about this, the more I think we're in a bit of pickle and
need Peter Maydell to chime in. While we may want to make our -smp command
line option parsing more strict in order to bring some sanity to it, if
we do, then we'll break existing command lines, which, while may be
specifying useless inputs, have always gotten away with it. We probably
can't just change that now without forcing the user to opt into it.
Maybe we need to add another -smp parameter like 'strict' that has to
be set to 'on' in order to get this new behavior.

Peter, do you have some suggestions for this? A summary of the problem
we'd like to solve is as follows:

 We'd like to start describing CPU topology to guests when provided
 topology information with the '-smp ...' command line option. Currently,
 a user may provide nearly whatever it wants on that command line option
 and not get an error, even though the guest will not get a topology
 description. When building the topology its important to know what
 the user actually wants, so we're proposing to require both sockets
 and cores be given if one of them is given. Also, since we don't yet
 support hot plug for AArch64, we're proposing to enforce cpus == maxcpus.

Is it fine to make those changes to the parsing for 6.1 and later? (Note,
mach-virt will override the default smp_parse with its own, so this is
mach-virt specific.) Or, should we only do this if a new parameter is
also given, e.g. 'strict'. Something like

  -smp strict=on,cpus=4,sockets=2,cores=2

would be needed by users who want to describe cpu topologies. Without
a strict description, then they get what they get today for their
DT/ACPI topology description, nothing.

Thanks,
drew