ppc/xive2: Set CPPR delivery should account for group priority

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>
Reviewed-by: Michael Kowal <kowal@linux.ibm.com>
Reviewed-by: Caleb Schlossin <calebs@linux.ibm.com>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20250512031100.439842-13-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

1 files changed, 22 insertions, 10 deletions

diff --git a/hw/intc/xive2.c b/hw/intc/xive2.c
index 181d1ae..cca121b 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);
 }