Re: [PATCH v6 04/15] xen/arm: add Dom0 cache coloring support

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.02.2024 14:35, Julien Grall wrote:
> Hi Jan,
> 
> On 01/02/2024 13:30, Jan Beulich wrote:
>> On 29.01.2024 18:18, Carlo Nonato wrote:
>>> Add a command line parameter to allow the user to set the coloring
>>> configuration for Dom0.
>>> A common configuration syntax for cache colors is introduced and
>>> documented.
>>> Take the opportunity to also add:
>>>   - default configuration notion.
>>>   - function to check well-formed configurations.
>>>
>>> Direct mapping Dom0 isn't possible when coloring is enabled, so
>>> CDF_directmap flag is removed when creating it.
>>
>> What implications does this have?
>>
>>> --- a/docs/misc/xen-command-line.pandoc
>>> +++ b/docs/misc/xen-command-line.pandoc
>>> @@ -963,6 +963,15 @@ Controls for the dom0 IOMMU setup.
>>>   
>>>   Specify a list of IO ports to be excluded from dom0 access.
>>>   
>>> +### dom0-llc-colors
>>> +> `= List of [ <integer> | <integer>-<integer> ]`
>>> +
>>> +> Default: `All available LLC colors`
>>> +
>>> +Specify dom0 LLC color configuration. This options is available only when
>>> +`CONFIG_LLC_COLORING` is enabled. If the parameter is not set, all 
>>> available
>>> +colors are used.
>>
>> Even Arm already has a "dom0=" option. Is there a particular reason why
>> this doesn't become a new sub-option there?
>>
>> As to meaning: With just a single <integer>, that's still a color value
>> then (and not a count of colors)? Wouldn't it make sense to have a
>> simpler variant available where you just say how many, and a suitable
>> set/range is then picked?
>>
>> Finally a nit: "This option is ...".
>>
>>> @@ -2188,10 +2190,16 @@ void __init create_dom0(void)
>>>               panic("SVE vector length error\n");
>>>       }
>>>   
>>> -    dom0 = domain_create(0, &dom0_cfg, CDF_privileged | CDF_directmap);
>>> +    if ( !llc_coloring_enabled )
>>> +        flags |= CDF_directmap;
>>> +
>>> +    dom0 = domain_create(0, &dom0_cfg, flags);
>>>       if ( IS_ERR(dom0) )
>>>           panic("Error creating domain 0 (rc = %ld)\n", PTR_ERR(dom0));
>>>   
>>> +    if ( llc_coloring_enabled && (rc = dom0_set_llc_colors(dom0)) )
>>> +        panic("Error initializing LLC coloring for domain 0 (rc = %d)", 
>>> rc);
>>
>> As for the earlier patch, I find panic()ing here dubious. You can continue
>> quite fine, with a warning and perhaps again tainting the system.
> There are arguments for both approach.

In which case - perhaps allow for both? With a Kconfig-established
default and a command line option to override?

> I agree that you can continue but 
> technically this is not the configuration you asked. Someone may not 
> notice the tainting until it is too late (read they have done 
> investigation).
> 
> Bear in mind that the user for cache coloring will be in very 
> specialized environment.

s/will/may/ I suppose. People may enable the option without being in
any specialized environment.

> So if you can't enable cache coloring in 
> production, then something really wrong has happened and continue to 
> boot is probably not right.
> 
> This matches the approach for Arm we have been using since the 
> beginning. And I will strongly argue to continue this way.

I'm okay with this, and here (for Arm-specific code) it may even be okay
to do so without further justification. But in the earlier patch where
common code is affected, I'll insist on at least justifying this behavior.

Jan