Re: [PATCH v7 01/14] xen/common: add cache coloring common code

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.03.2024 16:03, Carlo Nonato wrote:
> On Tue, Mar 19, 2024 at 3:58 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> On 15.03.2024 11:58, Carlo Nonato wrote:
>>> --- a/docs/misc/xen-command-line.pandoc
>>> +++ b/docs/misc/xen-command-line.pandoc
>>> @@ -1706,6 +1706,43 @@ This option is intended for debugging purposes only. 
>>>  Enable MSR_DEBUGCTL.LBR
>>>  in hypervisor context to be able to dump the Last Interrupt/Exception 
>>> To/From
>>>  record with other registers.
>>>
>>> +### llc-coloring
>>> +> `= <boolean>`
>>> +
>>> +> Default: `false`
>>> +
>>> +Flag to enable or disable LLC coloring support at runtime. This option is
>>> +available only when `CONFIG_LLC_COLORING` is enabled. See the general
>>> +cache coloring documentation for more info.
>>> +
>>> +### llc-nr-ways
>>> +> `= <integer>`
>>> +
>>> +> Default: `Obtained from hardware`
>>> +
>>> +Specify the number of ways of the Last Level Cache. This option is 
>>> available
>>> +only when `CONFIG_LLC_COLORING` is enabled. LLC size and number of ways 
>>> are used
>>> +to find the number of supported cache colors. By default the value is
>>> +automatically computed by probing the hardware, but in case of specific 
>>> needs,
>>> +it can be manually set. Those include failing probing and debugging/testing
>>> +purposes so that it's possibile to emulate platforms with different number 
>>> of
>>> +supported colors. If set, also "llc-size" must be set, otherwise the 
>>> default
>>> +will be used.
>>> +
>>> +### llc-size
>>> +> `= <size>`
>>> +
>>> +> Default: `Obtained from hardware`
>>> +
>>> +Specify the size of the Last Level Cache. This option is available only 
>>> when
>>> +`CONFIG_LLC_COLORING` is enabled. LLC size and number of ways are used to 
>>> find
>>> +the number of supported cache colors. By default the value is automatically
>>> +computed by probing the hardware, but in case of specific needs, it can be
>>> +manually set. Those include failing probing and debugging/testing purposes 
>>> so
>>> +that it's possibile to emulate platforms with different number of supported
>>> +colors. If set, also "llc-nr-ways" must be set, otherwise the default will 
>>> be
>>> +used.
>>
>> Wouldn't it make sense to infer "llc-coloring" when both of the latter 
>> options
>> were supplied?
> 
> To me it looks a bit strange that specifying some attributes of the cache
> automatically enables cache coloring. Also it would require some changes in
> how to express the auto-probing for such attributes.

Whereas to me it looks strange that, when having llc-size and llc-nr-ways
provided, I'd need to add a 3rd option. What purpose other than enabling
coloring could there be when specifying those parameters?

Jan