Re: [Xen-devel] [PATCH 4/5] x86: allow limiting the max C-state sub-state

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/05/2019 13:18, Jan Beulich wrote:
> From: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
>
> Allow limiting the max C-state sub-state by appending to the max_cstate
> command-line parameter. E.g. max_cstate=1,0
> The limit only applies to the highest legal C-state. For example:
>  max_cstate = 1, max_csubstate = 0 ==> C0, C1 okay, but not C1E
>  max_cstate = 1, max_csubstate = 1 ==> C0, C1 and C1E okay, but not C2
>  max_cstate = 2, max_csubstate = 0 ==> C0, C1, C1E, C2 okay, but not C3
>  max_cstate = 2, max_csubstate = 1 ==> C0, C1, C1E, C2 okay, but not C3
>
> Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Is this only useful in practice for limiting C1E ?

>
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -1369,9 +1369,11 @@ Specify the maximum number of CPUs that
>  This option is ignored in **pv-shim** mode.
>  
>  ### max_cstate (x86)
> -> `= <integer>`
> +> `= <integer>[,<integer>]`
>  
> -Specify the deepest C-state CPUs are permitted to be placed in.
> +Specify the deepest C-state CPUs are permitted to be placed in, and
> +optionally the maximum sub C-state to be used used.  The latter only applies
> +to the highest permitted C-state.
>  
>  ### max_gsi_irqs (x86)
>  > `= <integer>`
> --- a/xen/arch/x86/acpi/cpu_idle.c
> +++ b/xen/arch/x86/acpi/cpu_idle.c
> @@ -104,7 +104,17 @@ bool lapic_timer_init(void)
>  
>  void (*__read_mostly pm_idle_save)(void);
>  unsigned int max_cstate __read_mostly = UINT_MAX;
> -integer_param("max_cstate", max_cstate);
> +unsigned int max_csubstate __read_mostly = UINT_MAX;
> +
> +static int __init parse_cstate(const char *s)
> +{
> +    max_cstate = simple_strtoul(s, &s, 0);
> +    if ( *s == ',' )
> +        max_csubstate = simple_strtoul(s + 1, &s, 0);

You can pass NULL for endp, seeing as it isn't used.

~Andrew

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