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

[Carlo Nonato <carlo.nonato@xxxxxxxxxxxxxxx>]

Hi Jan,

On Thu, Mar 21, 2024 at 4:53 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> 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?

Ok, I probably misunderstood you. You mean just to assume llc-coloring=on
when both llc-size and llc-nr-ways are present and not to remove
llc-coloring completely, right? I'm ok with this.

> Jan

Thanks.