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By default, 'intel_idle' driver > enables C1 and disables C1E. However, some users prefer to use C1E instead of > C1, because it saves more energy. > > This patch adds a new module parameter ('preferred_cstates') for enabling C1E > and disabling C1. Here is the idea behind it. > > 1. This option has effect only for "mutually exclusive" C-states like C1 and > C1E on SPR. > 2. It does not have any effect on independent C-states, which do not require > other C-states to be disabled (most states on most platforms as of today). > 3. For mutually exclusive C-states, the 'intel_idle' driver always has a > reasonable default, such as enabling C1 on SPR by default. On other > platforms, the default may be different. > 4. Users can override the default using the 'preferred_cstates' parameter. > 5. The parameter accepts the preferred C-states bit-mask, similarly to the > existing 'states_off' parameter. > 6. This parameter is not limited to C1/C1E, and leaves room for supporting > other mutually exclusive C-states, if they come in the future. > > Today 'intel_idle' can only be compiled-in, which means that on SPR, in order > to disable C1 and enable C1E, users should boot with the following kernel > argument: intel_idle.preferred_cstates=4 > > Signed-off-by: Artem Bityutskiy > Signed-off-by: Rafael J. Wysocki > Origin: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git da0e58c038e6 > > Enable C1E (if requested) not only on the BSP's socket / package. Maybe we should also add a note here that the command line option for Xen is preferred-cstates instead of intel_idle.preferred_cstates? I think this is a bad interface however, we should have a more generic option (ie: cstate-mode = 'performance | powersave') so that users don't have to fiddle with model specific C state masks. > > Signed-off-by: Jan Beulich > > --- unstable.orig/docs/misc/xen-command-line.pandoc 2022-04-25 17:59:42.123387258 +0200 > +++ unstable/docs/misc/xen-command-line.pandoc 2022-04-25 17:36:00.000000000 +0200 > @@ -1884,6 +1884,12 @@ paging controls access to usermode addre > ### ple_window (Intel) > > `= ` > > +### preferred-cstates (x86) > +> `= ` > + > +This is a mask of C-states which are to be use preferably. This option is > +applicable only oh hardware were certain C-states are exlusive of one another. > + > ### psr (Intel) > > `= List of ( cmt: | rmid_max: | cat: | cos_max: | cdp: )` > > --- unstable.orig/xen/arch/x86/cpu/mwait-idle.c 2022-04-25 17:17:05.000000000 +0200 > +++ unstable/xen/arch/x86/cpu/mwait-idle.c 2022-04-25 17:33:47.000000000 +0200 > @@ -82,6 +82,18 @@ boolean_param("mwait-idle", opt_mwait_id > > static unsigned int mwait_substates; > > +/* > + * Some platforms come with mutually exclusive C-states, so that if one is > + * enabled, the other C-states must not be used. Example: C1 and C1E on > + * Sapphire Rapids platform. This parameter allows for selecting the > + * preferred C-states among the groups of mutually exclusive C-states - the > + * selected C-states will be registered, the other C-states from the mutually > + * exclusive group won't be registered. If the platform has no mutually > + * exclusive C-states, this parameter has no effect. > + */ > +static unsigned int __ro_after_init preferred_states_mask; > +integer_param("preferred-cstates", preferred_states_mask); > + > #define LAPIC_TIMER_ALWAYS_RELIABLE 0xFFFFFFFF > /* Reliable LAPIC Timer States, bit 1 for C1 etc. Default to only C1. */ > static unsigned int lapic_timer_reliable_states = (1 << 1); > @@ -96,6 +108,7 @@ struct idle_cpu { > unsigned long auto_demotion_disable_flags; > bool byt_auto_demotion_disable_flag; > bool disable_promotion_to_c1e; > + bool enable_promotion_to_c1e; I'm confused by those fields, shouldn't we just have: promotion_to_c1e = true | false? As one field is the negation of the other: enable_promotion_to_c1e = !disable_promotion_to_c1e I know this is code from Linux, but would like to understand why two fields are needed. Thanks, Roger.