arm64 and riscv pmap_copy_page(9) use splbio(9) and document why it is
required.  The same apply to uvm_pmr_cache_* functions.

The diff below applies the same comment and document why those spl are
required.

ok?

Index: uvm/uvm_pmemrange.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_pmemrange.c,v
diff -u -p -r1.77 uvm_pmemrange.c
--- uvm/uvm_pmemrange.c	19 Feb 2025 11:10:54 -0000	1.77
+++ uvm/uvm_pmemrange.c	9 Nov 2025 10:44:19 -0000
@@ -2239,7 +2239,7 @@ uvm_pmr_cache_alloc(struct uvm_pmr_cache
 	int flags = UVM_PLA_NOWAIT|UVM_PLA_NOWAKE;
 	int npages = UVM_PMR_CACHEMAGSZ;
 
-	splassert(IPL_VM);
+	splassert(IPL_BIO);
 	KASSERT(upci->upci_npages == 0);
 
 	TAILQ_INIT(&pgl);
@@ -2264,7 +2264,11 @@ uvm_pmr_cache_get(int flags)
 	struct vm_page *pg;
 	int s;
 
-	s = splvm();
+	/*
+	 * XXX The buffer flipper (incorrectly?) allocates & frees pages
+	 * (from uvm_pagerealloc_multi()) from interrupt context!
+	 */
+	s = splbio();
 	upci = &upc->upc_magz[upc->upc_actv];
 	if (upci->upci_npages == 0) {
 		unsigned int prev;
@@ -2301,7 +2305,7 @@ uvm_pmr_cache_free(struct uvm_pmr_cache_
 	struct pglist pgl;
 	unsigned int i;
 
-	splassert(IPL_VM);
+	splassert(IPL_BIO);
 
 	TAILQ_INIT(&pgl);
 	for (i = 0; i < upci->upci_npages; i++)
@@ -2334,7 +2338,11 @@ uvm_pmr_cache_put(struct vm_page *pg)
 		return;
 	}
 
-	s = splvm();
+	/*
+	 * XXX The buffer flipper (incorrectly?) allocates & frees pages
+	 * (from uvm_pagerealloc_multi()) from interrupt context!
+	 */
+	s = splbio();
 	upci = &upc->upc_magz[upc->upc_actv];
 	if (upci->upci_npages >= UVM_PMR_CACHEMAGSZ) {
 		unsigned int prev;
@@ -2362,7 +2370,11 @@ uvm_pmr_cache_drain(void)
 	unsigned int freed = 0;
 	int s;
 
-	s = splvm();
+	/*
+	 * XXX The buffer flipper (incorrectly?) allocates & frees pages
+	 * (from uvm_pagerealloc_multi()) from interrupt context!
+	 */
+	s = splbio();
 	freed += uvm_pmr_cache_free(&upc->upc_magz[0]);
 	freed += uvm_pmr_cache_free(&upc->upc_magz[1]);
 	splx(s);