From 739924cb5cb84e8f29c90b4accd0910daa58a579 Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Sat, 20 Sep 2025 17:13:57 -0400
Subject: kern/amd64: mmu: Add page-level cacheability attrs

This commit adds support for page-level cacheability attributes. We have
added the pmap_set_cache() as well as some MMU_CACHE_* bits

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 src/sys/arch/amd64/cpu/mmu.c | 45 ++++++++++++++++++++++++++++++++++++++++++++
 src/sys/include/vm/mmu.h     | 23 ++++++++++++++++++++++
 2 files changed, 68 insertions(+)

diff --git a/src/sys/arch/amd64/cpu/mmu.c b/src/sys/arch/amd64/cpu/mmu.c
index 2b98d78..dad22dc 100644
--- a/src/sys/arch/amd64/cpu/mmu.c
+++ b/src/sys/arch/amd64/cpu/mmu.c
@@ -404,6 +404,51 @@ mmu_free_vas(struct vm_vas *vas)
     return 0;
 }
 
+/*
+ * Set caching attributes
+ */
+int
+pmap_set_cache(struct vm_vas *vas, vaddr_t va, cacheattr_t attr)
+{
+    uintptr_t *pte, pa;
+    uint32_t flags;
+    int error;
+    size_t idx;
+
+    /* We'll modify page-level attributes */
+    error = mmu_read_level(
+        vas, va, MMU_TBL,
+        &pte, true
+    );
+
+    if (error < 0) {
+        return -EINVAL;
+    }
+
+    /* Uncachable? */
+    if (ISSET(attr, MMU_CACHE_UC)) {
+        flags |= PTE_PCD;
+        flags &= ~PTE_PWT;
+        attr &= ~MMU_CACHE_WT;
+    }
+
+    /* Write through? */
+    if (ISSET(attr, MMU_CACHE_WT)) {
+        flags &= ~PTE_PCD;
+        flags |= PTE_PWT;
+    }
+
+    /* Not global? */
+    if (!ISSET(attr, MMU_CACHE_GL)) {
+        flags &= ~PTE_GLOBAL;
+    }
+
+    /* Update attributes and flush the TLB */
+    pte[idx] = pa | flags;
+    __invlpg((void *)va);
+    return 0;
+}
+
 /*
  * Verify that we are in a known state
  */
diff --git a/src/sys/include/vm/mmu.h b/src/sys/include/vm/mmu.h
index be0678e..54a694b 100644
--- a/src/sys/include/vm/mmu.h
+++ b/src/sys/include/vm/mmu.h
@@ -41,6 +41,17 @@
 #include <machine/vas.h>    /* standard */
 #include <vm/vm.h>
 
+/* Caching types */
+#define MMU_CACHE_UC BIT(0) /* Uncachable */
+#define MMU_CACHE_WT BIT(1) /* Write-through */
+#define MMU_CACHE_GL BIT(2) /* Global (if supported) */
+
+/*
+ * Represents caching attributes that can be applied
+ * to the address space (see VM_CACHE_* above).
+ */
+typedef uint16_t cacheattr_t;
+
 /*
  * Standard memory protection flags
  */
@@ -127,4 +138,16 @@ int mmu_write_vas(struct vm_vas *vas);
  */
 int mmu_free_vas(struct vm_vas *vas);
 
+/*
+ * Update page-level caching attributes
+ *
+ * @vas: Virtual address space to target
+ * @va: Virtual page base to update
+ * @attr: Attributes to set
+ *
+ * Returns zero on success, otherwise a less than
+ * zero value to indicate error.
+ */
+int pmap_set_cache(struct vm_vas *vas, vaddr_t va, cacheattr_t attr);
+
 #endif  /* !_MACHINE_MMU_H_ */
-- 
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