Re: [PATCH v2 2/2] qom/object, accel-system: add support to Accel globals

[Markus Armbruster <armbru@redhat.com>]

Andrew Jones <ajones@ventanamicro.com> writes:

> On Wed, Jul 31, 2024 at 08:30:46AM GMT, Markus Armbruster wrote:
>> I apologize for the delay.
>> 
>> Daniel Henrique Barboza <dbarboza@ventanamicro.com> writes:
>> 
>> > We're not honouring KVM options that are provided by any -accel option
>> > aside from the first. In this example:
>> >
>> > qemu-system-riscv64 -accel kvm,riscv-aia=emul (...) \
>> >     -accel kvm,riscv-aia=hwaccel
>> >
>> > 'riscv-aia' will be set to 'emul', ignoring the last occurrence of the
>> > option that set 'riscv-aia' to 'hwaccel'.
>> 
>> The way you phrase this, it sounds like a bug.  But as far as I know,
>> -accel is meant to have fallback semantics: we use the first one that
>> works.
>
> The fact that some (most?) parameters have override semantics and some
> have fallback semantics makes our complicated command line even more
> complicated, especially since there's no way to know which is which.

We traditionally tramsmit such knowledge from guru to disciple.

We certainly made an unholy mess of our command line.

> IMHO, always having override semantics and then providing new parameters,
> e.g. -accel-fallback (or a property, -accel fallback=on,...), would go a
> long way to bringing some order to the universe.
>
>> Perhaps:
>> 
>>   -accel has fallback semantics, i.e. we try accelerators in order until
>>   we find one that works.  Any remainder is ignored.
>> 
>>   Because of that, you can't override properties like this:
>> 
>>       qemu-system-riscv64 -accel kvm,riscv-aia=emul (...) \
>>           -accel kvm,riscv-aia=hwaccel
>> 
>>   When KVM is available, 'riscv-aia' will be set to 'emul', and the
>>   second -accel is ignored.  When KVM is not available, neither option
>>   works, and the command fails.
>> 
>> Why would you want to override accelerator properties?
>
> Testing. Many properties are only available to allow the user to force
> non defaults. The example above isn't exactly what triggered this. The
> real use is, '-accel kvm' is the default used by libvirt and when
> riscv-aia=hwaccel is possible, it will default to hwaccel. In order to
> test riscv-aia emulation support using libvirt (which doesn't yet allow
> selecting anything riscv specific), I attempted to use the qemu
> commandline element to override -accel with kvm,riscv-aia=emul.

Ah, that explains why -global solves your problem, too!  Thanks!

I recommend to start the commit message with the use case, then describe
the solution.  Mention other solutions last, if at all.