Re: x86 CPU properties "family", "model", "stepping", "tsc-freq"

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Oct 09, 2024 at 01:01:40PM +0200, Markus Armbruster wrote:
> QOM properties serve several purposes: initial configuration (external
> interface), run time control and monitoring (external interface), and
> internal purposes like versioning.  Which purpose(s) a property serves
> is often unclear.
> 
> The x86 CPU properties "family", "model", and "stepping" are used
> internally; see target/i386/cpu.c and hw/i386/pc.c.  I figure changing
> them at run time makes no sense.  What about configuration?  Can the
> user set arbitrary CPU properties?  If yes, are these properties meant
> to be set by the user?

The named CPU models have associated family/model/stepping (FMS),
and the string "model-i"' info defined against them, that match
the some arbitrary choice of silicon.

QEMU doesn't support all possible values though. For example,
we've seen real Skylake-Server CPUs with stepping in the range
0-4, but QEMU always reports 4 for stepping, so altering that
is potentially interesting if wanting to check guest behaviour
with a very specific FMS value.

More generally QEMU doesn't support every possible CPU model
that exists.

You can invent entirely new CPUs by turning on/off individual
CPUID feature flags, along with setting the FMS and the model-id
string.

 $ ./make-tiny-image.py --run  "cat /proc/cpuinfo"
  6.9.9-200.fc40.x86_64

 $ qemu-system-x86_64  -nodefaults -display none -serial stdio -accel kvm -m 1024 -kernel /lib/modules/6.10.8-200.fc40.x86_64/vmlinuz -initrd tiny-initrd.img  -append 'console=ttyS0 quiet' -cpu Skylake-Server | grep -E '(model|family|stepping)'
cpu family	: 6
model		: 85
model name	: Intel Xeon Processor (Skylake)
stepping	: 4

Invent my own CPU...

 $ qemu-system-x86_64  -nodefaults -display none -serial stdio -accel kvm -m 1024 -kernel /lib/modules/6.10.8-200.fc40.x86_64/vmlinuz -initrd tiny-initrd.img  -append 'console=ttyS0 quiet' -cpu Skylake-Server,family=13,model=42,model-id="Fish Food",stepping=3 | grep -E '(model|family|stepping)'
 cpu family	: 13
 model		: 42
 model name	: Fish Food
 stepping	: 1

Whether or not the requested FMS & model-id choices actually make
conceptual sense is upto the user to figure out :-)

Setting these values is certainly niche, but still valid IMHO.

> Property "tsc-freq" seems not to be used internally.  I figure changing
> it at run time makes no sense.  Looks like it is be meant to be set by
> the user: x86_cpu_parse_featurestr() seems to parse it from the argument
> of -cpu.  Correct?  Note that -cpu help doesn't mention it.

"tsc-freq" is a back compat alias for 'tsc-frequency' AFAICT, and
we need the latter if you want to guarantee fixed TSC freq across
migration. eg commit 561dbb41b1d752098249128d8462aaadc56fd15d

With regards,
Daniel
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