Re: [PATCH v2 2/5] public/sysctl: Clarify usage of pm_{px,cx}_stat

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.05.2025 16:46, Ross Lagerwall wrote:
> --- a/xen/include/public/sysctl.h
> +++ b/xen/include/public/sysctl.h
> @@ -215,23 +215,51 @@ typedef struct pm_px_val pm_px_val_t;
>  DEFINE_XEN_GUEST_HANDLE(pm_px_val_t);
>  
>  struct pm_px_stat {
> -    uint8_t total;        /* total Px states */
> -    uint8_t usable;       /* usable Px states */
> -    uint8_t last;         /* last Px state */
> -    uint8_t cur;          /* current Px state */
> -    XEN_GUEST_HANDLE_64(uint64) trans_pt;   /* Px transition table */
> +    /*
> +     * IN: Number of elements in pt, number of rows/columns in trans_pt
> +     *     (PMSTAT_get_pxstat)
> +     * OUT: total Px states (PMSTAT_get_max_px, PMSTAT_get_pxstat)
> +     */
> +    uint8_t total;

The part for this field ought to go in patch 1, as already indicated there.

> +    uint8_t usable;       /* OUT: usable Px states (PMSTAT_get_pxstat) */
> +    uint8_t last;         /* OUT: last Px state (PMSTAT_get_pxstat) */
> +    uint8_t cur;          /* OUT: current Px state (PMSTAT_get_pxstat) */
> +    /*
> +     * OUT: Px transition table. This should have total * total elements.
> +     *      This will not be copied if it is smaller than the hypervisor's
> +     *      Px transition table. (PMSTAT_get_pxstat)
> +     */
> +    XEN_GUEST_HANDLE_64(uint64) trans_pt;
> +    /* OUT: This should have total elements (PMSTAT_get_pxstat) */
>      XEN_GUEST_HANDLE_64(pm_px_val_t) pt;

As also indicated there, the same constraint as for trans_pt applies to this
output buffer, just that it's having only one dimension.

>  };
>  
>  struct pm_cx_stat {
> -    uint32_t nr;    /* entry nr in triggers & residencies, including C0 */
> -    uint32_t last;  /* last Cx state */
> -    uint64_aligned_t idle_time;                 /* idle time from boot */
> -    XEN_GUEST_HANDLE_64(uint64) triggers;    /* Cx trigger counts */
> -    XEN_GUEST_HANDLE_64(uint64) residencies; /* Cx residencies */
> -    uint32_t nr_pc;                          /* entry nr in pc[] */
> -    uint32_t nr_cc;                          /* entry nr in cc[] */
>      /*
> +     * IN:  Number of elements in triggers, residencies (PMSTAT_get_cxstat)
> +     * OUT: entry nr in triggers & residencies, including C0
> +     *      (PMSTAT_get_cxstat, PMSTAT_get_max_cx)
> +     */
> +    uint32_t nr;
> +    uint32_t last;  /* OUT: last Cx state (PMSTAT_get_cxstat) */
> +    /* OUT: idle time from boot (PMSTAT_get_cxstat)*/
> +    uint64_aligned_t idle_time;
> +    /* OUT: Cx trigger counts, nr elements (PMSTAT_get_cxstat) */
> +    XEN_GUEST_HANDLE_64(uint64) triggers;
> +    /* OUT: Cx residencies, nr elements (PMSTAT_get_cxstat) */
> +    XEN_GUEST_HANDLE_64(uint64) residencies;
> +    /*
> +     * IN: entry nr in pc[] (PMSTAT_get_cxstat)
> +     * OUT: Index of highest non-zero entry set in pc[] (PMSTAT_get_cxstat)
> +     */
> +    uint32_t nr_pc;
> +    /*
> +     * IN: entry nr in cc[] (PMSTAT_get_cxstat)
> +     * OUT: Index of highest non-zero entry set in cc[] (PMSTAT_get_cxstat)
> +     */

For both of these, it's not "highest non-zero" but, according to ...

> +    uint32_t nr_cc;
> +    /*
> +     * OUT: (PMSTAT_get_cxstat)
>       * These two arrays may (and generally will) have unused slots; slots not
>       * having a corresponding hardware register will not be written by the
>       * hypervisor. It is therefore up to the caller to put a suitable 
> sentinel

... this comment, "highest written by hypervisor". They're also not "index of",
but "one higher than the index of" (i.e. counts, not indexes).

Jan