Re: [PATCH 14/15] x86/hyperlaunch: add max vcpu parsing of hyperlaunch device tree

[Jan Beulich <jbeulich@suse.com>]

On 11.12.2024 20:49, Daniel P. Smith wrote:
> On 12/2/24 07:19, Jan Beulich wrote:
>> On 23.11.2024 19:20, Daniel P. Smith wrote:
>>> --- a/xen/arch/x86/dom0_build.c
>>> +++ b/xen/arch/x86/dom0_build.c
>>> @@ -617,6 +617,9 @@ int __init construct_dom0(struct boot_domain *bd)
>>>       if ( !get_memsize(&dom0_max_size, LONG_MAX) && bd->max_pages )
>>>           dom0_size.nr_pages = bd->max_pages;
>>>   
>>> +    if ( opt_dom0_max_vcpus_max == UINT_MAX && bd->max_vcpus )
>>> +        opt_dom0_max_vcpus_max = bd->max_vcpus;
>>
>> Isn't this kind of backwards? I.e. aren't you meaning to move us towards
>> boot-domains?
> 
> Prior to domain builder, available construction parameters for dom0 were 
> exposed as command line parameters. This allowed for boot-time 
> adjustments to the parameters. With domain builder, there are now two 
> sources for dom0 construction parameters. Those coming from the device 
> tree and those coming from the command line. For most x86 platforms, the 
> device tree parameters can only be constructed prior to booting Xen. 
> Whereas the command line parameters allow boot-time adjustments, at 
> least for dom0. That is the thinking at least. Now if there is interest 
> in being able to retire the command line options, that would definitely 
> simplify things.

No, retiring command line options is out of question imo. Yet that also
wasn't my point. Instead I was wondering why we wouldn't make bd->* the
ultimate source of truth. However, ...

>> Also, what about the counterpart opt_dom0_max_vcpus_min? That wants to be
>> controllable from DT too, I would think?
> 
> Yes, in theory we will eventually be able to do requested/min/max as 
> well as cpu pinning/affinity. For now it was requested we focus on 
> implementing only requested vcpus.

... that's pretty much only a reasonable option if these were converted
at the same time, to avoid becoming inconsistent for perhaps an extended
period of time.

Jan