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diff --git a/sys/riscv/riscv/pmap.c b/sys/riscv/riscv/pmap.c
--- a/sys/riscv/riscv/pmap.c
+++ b/sys/riscv/riscv/pmap.c
@@ -279,6 +279,13 @@
&pmap_l2_promotions, 0,
"2MB page promotions");
+static SYSCTL_NODE(_vm_pmap, OID_AUTO, l1, CTLFLAG_RD | CTLFLAG_MPSAFE, 0,
+ "L1 (1GB) page mapping counters");
+
+static COUNTER_U64_DEFINE_EARLY(pmap_l1_demotions);
+SYSCTL_COUNTER_U64(_vm_pmap_l1, OID_AUTO, demotions, CTLFLAG_RD,
+ &pmap_l1_demotions, "L1 (1GB) page demotions");
+
/*
* Data for the pv entry allocation mechanism
*/
@@ -303,6 +310,7 @@
static void pmap_pvh_free(struct md_page *pvh, pmap_t pmap, vm_offset_t va);
static pv_entry_t pmap_pvh_remove(struct md_page *pvh, pmap_t pmap,
vm_offset_t va);
+static bool pmap_demote_l1(pmap_t pmap, pd_entry_t *l1, vm_offset_t va);
static bool pmap_demote_l2(pmap_t pmap, pd_entry_t *l2, vm_offset_t va);
static bool pmap_demote_l2_locked(pmap_t pmap, pd_entry_t *l2,
vm_offset_t va, struct rwlock **lockp);
@@ -2795,6 +2803,67 @@
return (rv);
}
+/*
+ * Demote the specified L1 page to separate L2 pages.
+ * Currently only used for DMAP entries.
+ */
+static bool
+pmap_demote_l1(pmap_t pmap, pd_entry_t *l1, vm_offset_t va)
+{
+ vm_page_t m;
+ pt_entry_t *l2, oldl1, newl2;
+ pd_entry_t newl1;
+ vm_paddr_t l2phys;
+
+ PMAP_LOCK_ASSERT(pmap, MA_OWNED);
+
+ oldl1 = pmap_load(l1);
+ KASSERT((oldl1 & PTE_RWX) != 0,
+ ("pmap_demote_l1: oldl1 is not a leaf PTE"));
+ KASSERT((oldl1 & PTE_A) != 0,
+ ("pmap_demote_l1: oldl1 is missing PTE_A"));
+ KASSERT((oldl1 & (PTE_D | PTE_W)) != PTE_W,
+ ("pmap_demote_l1: not dirty!"));
+ KASSERT((oldl1 & PTE_SW_MANAGED) == 0,
+ ("pmap_demote_l1: L1 table shouldn't be managed"));
+ KASSERT(VIRT_IN_DMAP(va),
+ ("pmap_demote_l1: is unsupported for non-DMAP va=%#lx", va));
+
+ /* Demoting L1 means we need to allocate a new page-table page. */
+ m = vm_page_alloc_noobj(VM_ALLOC_INTERRUPT | VM_ALLOC_WIRED);
+ if (m == NULL) {
+ CTR2(KTR_PMAP, "pmap_demote_l1: failure for va %#lx in pmap %p",
+ va, pmap);
+ return (false);
+ }
+
+ l2phys = VM_PAGE_TO_PHYS(m);
+ l2 = (pt_entry_t *)PHYS_TO_DMAP(l2phys);
+
+ /*
+ * Create new entries, relying on the fact that only the low bits
+ * (index) of the physical address are changing.
+ */
+ newl2 = oldl1;
+ for (int i = 0; i < Ln_ENTRIES; i++)
+ pmap_store(&l2[i], newl2 | (i << PTE_PPN1_S));
+
+ /*
+ * And update the L1 entry.
+ *
+ * NB: flushing the TLB is the responsibility of the caller. Cached
+ * translations are still "correct" for demoted mappings until some
+ * subset of the demoted range is modified.
+ */
+ newl1 = ((l2phys / PAGE_SIZE) << PTE_PPN0_S) | PTE_V;
+ pmap_store(l1, newl1);
+
+ counter_u64_add(pmap_l1_demotions, 1);
+ CTR2(KTR_PMAP, "pmap_demote_l1: success for va %#lx in pmap %p",
+ va, pmap);
+ return (true);
+}
+
static bool
pmap_demote_l2(pmap_t pmap, pd_entry_t *l2, vm_offset_t va)
{

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