Re: [PATCH 12/50] ppc/xive2: Set CPPR delivery should account for group priority

[Miles Glenn <milesg@linux.ibm.com>]

On Mon, 2025-05-12 at 13:10 +1000, Nicholas Piggin wrote:
> The group interrupt delivery flow selects the group backlog scan if
> LSMFB < IPB, but that scan may find an interrupt with a priority >=
> IPB. In that case, the VP-direct interrupt should be chosen. This
> extends to selecting the lowest prio between POOL and PHYS rings.
> 
> Implement this just by re-starting the selection logic if the
> backlog irq was not found or priority did not match LSMFB (LSMFB
> is updated so next time around it would see the right value and
> not loop infinitely).
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  hw/intc/xive2.c | 32 ++++++++++++++++++++++----------
>  1 file changed, 22 insertions(+), 10 deletions(-)
> 
> diff --git a/hw/intc/xive2.c b/hw/intc/xive2.c
> index 8ede95b671..de139dcfbf 100644
> --- a/hw/intc/xive2.c
> +++ b/hw/intc/xive2.c
> @@ -939,7 +939,7 @@ static void xive2_tctx_set_cppr(XiveTCTX *tctx, uint8_t ring, uint8_t cppr)
>  {
>      uint8_t *regs = &tctx->regs[ring];
>      Xive2Router *xrtr = XIVE2_ROUTER(tctx->xptr);
> -    uint8_t old_cppr, backlog_prio, first_group, group_level = 0;
> +    uint8_t old_cppr, backlog_prio, first_group, group_level;
>      uint8_t pipr_min, lsmfb_min, ring_min;
>      bool group_enabled;
>      uint32_t nvp_blk, nvp_idx;
> @@ -961,10 +961,12 @@ static void xive2_tctx_set_cppr(XiveTCTX *tctx, uint8_t ring, uint8_t cppr)
>       * Recompute the PIPR based on local pending interrupts. It will
>       * be adjusted below if needed in case of pending group interrupts.
>       */
> +again:
>      pipr_min = xive_ipb_to_pipr(regs[TM_IPB]);
>      group_enabled = !!regs[TM_LGS];
> -    lsmfb_min = (group_enabled) ? regs[TM_LSMFB] : 0xff;
> +    lsmfb_min = group_enabled ? regs[TM_LSMFB] : 0xff;
>      ring_min = ring;
> +    group_level = 0;
>  
>      /* PHYS updates also depend on POOL values */
>      if (ring == TM_QW3_HV_PHYS) {
> @@ -998,9 +1000,6 @@ static void xive2_tctx_set_cppr(XiveTCTX *tctx, uint8_t ring, uint8_t cppr)
>          }
>      }
>  
> -    /* PIPR should not be set to a value greater than CPPR */
> -    regs[TM_PIPR] = (pipr_min > cppr) ? cppr : pipr_min;
> -
>      rc = xive2_tctx_get_nvp_indexes(tctx, ring_min, &nvp_blk, &nvp_idx);
>      if (rc) {
>          qemu_log_mask(LOG_GUEST_ERROR, "XIVE: set CPPR on invalid context\n");
> @@ -1019,7 +1018,7 @@ static void xive2_tctx_set_cppr(XiveTCTX *tctx, uint8_t ring, uint8_t cppr)
>  
>      if (group_enabled &&
>          lsmfb_min < cppr &&
> -        lsmfb_min < regs[TM_PIPR]) {
> +        lsmfb_min < pipr_min) {
>          /*
>           * Thread has seen a group interrupt with a higher priority
>           * than the new cppr or pending local interrupt. Check the
> @@ -1048,12 +1047,25 @@ static void xive2_tctx_set_cppr(XiveTCTX *tctx, uint8_t ring, uint8_t cppr)
>                                                      nvp_blk, nvp_idx,
>                                                      first_group, &group_level);
>          tctx->regs[ring_min + TM_LSMFB] = backlog_prio;
> -        if (backlog_prio != 0xFF) {
> -            xive2_presenter_backlog_decr(tctx->xptr, nvp_blk, nvp_idx,
> -                                         backlog_prio, group_level);
> -            regs[TM_PIPR] = backlog_prio;
> +        if (backlog_prio != lsmfb_min) {
> +            /*
> +             * If the group backlog scan finds a less favored or no interrupt,
> +             * then re-do the processing which may turn up a more favored
> +             * interrupt from IPB or the other pool. Backlog should not
> +             * find a priority < LSMFB.
> +             */
> +            g_assert(backlog_prio >= lsmfb_min);
> +            goto again;
>          }
> +
> +        xive2_presenter_backlog_decr(tctx->xptr, nvp_blk, nvp_idx,
> +                                     backlog_prio, group_level);
> +        pipr_min = backlog_prio;
>      }
> +
> +    /* PIPR should not be set to a value greater than CPPR */
> +    regs[TM_PIPR] = (pipr_min > cppr) ? cppr : pipr_min;
> +
>      /* CPPR has changed, check if we need to raise a pending exception */
>      xive_tctx_notify(tctx, ring_min, group_level);
>  }