Re: [PATCH] x86: Conditionalise init_dom0_cpu_policy()

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

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.

Jan