Re: [PATCH] x86: Conditionalise init_dom0_cpu_policy()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.07.2025 11:48, Alejandro Vallejo wrote:
> On Wed Jul 30, 2025 at 9:48 AM CEST, Jan Beulich wrote:
>> On 29.07.2025 23:29, Daniel P. Smith wrote:
>>> On 7/25/25 06:56, Roger Pau Monné wrote:
>>>> On Fri, Jul 25, 2025 at 12:02:18PM +0200, Alejandro Vallejo wrote:
>>>>> On Wed Jul 23, 2025 at 9:18 AM CEST, Roger Pau Monné wrote:
>>>>>> On Thu, Jul 17, 2025 at 07:58:24PM +0200, Alejandro Vallejo wrote:
>>>>>>> Later patches will keep refactoring create_dom0()
>>>>>>> until it can create arbitrary domains. This is one
>>>>>>> small step in that direction.
>>>>>>>
>>>>>>> Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
>>>>>>> ---
>>>>>>>   xen/arch/x86/setup.c | 3 ++-
>>>>>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
>>>>>>> index c6890669b9..6943ffba79 100644
>>>>>>> --- a/xen/arch/x86/setup.c
>>>>>>> +++ b/xen/arch/x86/setup.c
>>>>>>> @@ -1054,7 +1054,8 @@ static struct domain *__init create_dom0(struct 
>>>>>>> boot_info *bi)
>>>>>>>       if ( IS_ERR(d) )
>>>>>>>           panic("Error creating d%u: %ld\n", bd->domid, PTR_ERR(d));
>>>>>>>   
>>>>>>> -    init_dom0_cpuid_policy(d);
>>>>>>> +    if ( pv_shim || d->cdf & (CDF_privileged | CDF_hardware) )
>>>>>>
>>>>>> You possibly want this to be:
>>>>>>
>>>>>> (d->cdf & (CDF_privileged | CDF_hardware)) == (CDF_privileged | 
>>>>>> CDF_hardware)
>>>>>>
>>>>>> To ensure the contents of dom0_cpuid_cmdline is only applied to dom0,
>>>>>> and not to the hardware or control domains.  I assume it should be
>>>>>> possible to pass a different set of cpuid options for the hardware vs
>>>>>> the control domains.
>>>>>>
>>>>>> Thanks, Roger.
>>>>>
>>>>> Why only a hwdom+ctldom, surely a single hwdom should get it too.
>>>>
>>>> hm, not really I think: a late hardware domain would get any custom
>>>> cpuid options from the toolstack that created it, or in the
>>>> hyperlaunch case from the provided configuration, but not from the
>>>> dom0-cpuid command line option I would expect.  Otherwise you have two
>>>> different sources of cpuid options, the inheritance from dom0-cpuid,
>>>> plus whatever is provided from the hardware domain configuration.
>>>
>>> Yes, this has been a sticking point for me and never got any good 
>>> answers thus far. Should the dom0 related xen command line options only 
>>> apply when not booting via hyperlaunch. If the answer is no, then you're 
>>> in this area with some dom0 options that really are applicable to hwdom 
>>> vs ctldom and vice-a-versa. Some could even be suggested to apply to 
>>> both. And then, I don't believe there really is a consensus one which 
>>> options apply to which domains. Over the years working on this, I have 
>>> been an advocate that commandline adjustments allow for quicker 
>>> troubleshooting by the user/administrator.
> 
>>> In the last version of the multidomain construction RFC, I am growing more
>>> and more to advocate for my initial proposition, that dom0 options only
>>> apply when not using  hyperlaunch.
> 
> I agree. It simplifies everything a ton, and it's far less confusing to know
> ultimate settings, which in a predefined initial system definition is 
> important.
> 
>>
>> With the hyperlaunch plans, is there something that's still properly
>> "Dom0", perhaps under certain conditions? That (and only that) is
>> where I would see respective command line options to apply. IOW no
>> more than one specific domain (i.e. in particular not to both hwdom
>> and ctldom, when they're separate). In cases when respective options
>> are entirely ignored, I think some kind of warning would want issuing.
> 
> The problem is that lines are blurred. A ctldtdom + hwdom + xsdom with domid0
> is clearly a dom0. Is it still a dom0 when there's no xenstore? What about 
> when
> it's not privileged? What about a ctldom + hwdom + xsdom with domid3? What 
> about
> dom0_mem options when some domains have already been constructed and available
> memory is less than total host memory?

Well, this is what needs determining before we actually move in any (unclear)
direction. And we need to keep in mind that people used to infer certain
things from domain ID being 0. 

> Also if a domain is or isn't dom0 depending on whether a certain other domain
> exists makes things confusing. You have a DTB+commandline and get a behaviour,
> then add a domain and you get another behaviour on the first one, even when 
> you
> didn't touch its configuration.
> 
> My general view after a while experimenting with the full series is to _not_ 
> use
> the dom0 command line, as Daniel mentions. The simplifying effect of not 
> looking
> at (e.g) dom0_mem is staggering.

Which likely would imply not to create any domain with ID 0.

Jan

> There's exceptions. nmi=dom0 should be renamed to nmi=hwdom (if anything,
> because that's exactly what it does even with late hwdom), but anything with
> dom0_X ought to be ignored. Which implies first and foremost moving its uses
> outside domain construction and general use.
> 
> All dom0_ options ought to be parsed and used from __init functions before
> construct_dom0(), and construct_dom0 ought to depend strictly on information
> in boot_domain + domain.
> 
> Only then we'll have sanity.
> 
> Cheers
> Alejandro