From 2c17a4db1d32a9ace5bdb430a0faf7e28a00110e Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Tue, 16 Sep 2025 00:29:14 -0400
Subject: kern/amd64: mmu: Add mmu_new_vas() routine

This commit introduces the ability to create new virtual address spaces
for user programs and a zeroed lower half.

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 src/sys/arch/amd64/cpu/mmu.c | 32 ++++++++++++++++++++++++++++++++
 src/sys/include/vm/mmu.h     |  9 +++++++++
 2 files changed, 41 insertions(+)

diff --git a/src/sys/arch/amd64/cpu/mmu.c b/src/sys/arch/amd64/cpu/mmu.c
index 60fb4f1..05e8b55 100644
--- a/src/sys/arch/amd64/cpu/mmu.c
+++ b/src/sys/arch/amd64/cpu/mmu.c
@@ -332,6 +332,38 @@ mmu_map_single(struct vm_vas *vas, struct mmu_map *spec, int prot)
     return 0;
 }
 
+/*
+ * Create a new virtual address space
+ */
+int
+mmu_new_vas(struct vm_vas *res)
+{
+    uint64_t *dest, *src;
+    paddr_t frame;
+
+    src = PHYS_TO_VIRT(g_kvas.cr3 & PTE_ADDR_MASK);
+    frame = vm_alloc_frame(1);
+    if (frame == 0) {
+        return -ENOMEM;
+    }
+
+    /*
+     * Copy only the higher half but zero the
+     * lower half.
+     */
+    dest = PHYS_TO_VIRT(frame);
+    for (int i = 0; i < 512; ++i) {
+        if (i < 256) {
+            dest[i] = 0;
+        } else {
+            dest[i] = src[i];
+        }
+    }
+
+    res->cr3 = VIRT_TO_PHYS(dest);
+    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 5c1c2bb..908748b 100644
--- a/src/sys/include/vm/mmu.h
+++ b/src/sys/include/vm/mmu.h
@@ -97,4 +97,13 @@ int mmu_map_single(struct vm_vas *vas, struct mmu_map *spec, int prot);
  */
 int mmu_this_vas(struct vm_vas *vasres_p);
 
+/*
+ * Create a new virtual address structure with a zeroed
+ * user porition.
+ *
+ * Returns zero on success, otherwise a less than zero
+ * value on failure.
+ */
+int mmu_new_vas(struct vm_vas *res);
+
 #endif  /* !_MACHINE_MMU_H_ */
-- 
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