From f3fe5e8d39fc9bbcffb346d0b7329445140a1e93 Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Tue, 18 Nov 2025 00:43:20 -0500
Subject: kern/amd64: mmu: Add mu_pmap_forkvas() helper

The mu_pmap_forkvas() function copies the current virtual address with
the user porition zeroed.

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 sys/arch/amd64/cpu/mmu.c | 28 ++++++++++++++++++++++++++++
 sys/inc/mu/mmu.h         |  6 ++++++
 sys/mu/mmu_stub.c        |  6 ++++++
 3 files changed, 40 insertions(+)

diff --git a/sys/arch/amd64/cpu/mmu.c b/sys/arch/amd64/cpu/mmu.c
index 65c4e46..15ba19a 100644
--- a/sys/arch/amd64/cpu/mmu.c
+++ b/sys/arch/amd64/cpu/mmu.c
@@ -28,10 +28,12 @@
  */
 
 #include <sys/types.h>
+#include <sys/errno.h>
 #include <sys/cdefs.h>
 #include <kern/panic.h>
 #include <mu/mmu.h>
 #include <vm/vm.h>
+#include <vm/phys.h>
 #include <md/vas.h>
 
 /*
@@ -76,6 +78,32 @@ mu_pmap_writevas(struct mmu_vas *vas)
     return 0;
 }
 
+int
+mu_pmap_forkvas(struct mmu_vas *result)
+{
+    struct mmu_vas vas;
+    uintptr_t paddr, *pml4_dest;
+    uintptr_t *pml4_src;
+
+    mu_pmap_readvas(&vas);
+    paddr = vm_phys_alloc(1);
+    if (paddr == 0) {
+        return -ENOMEM;
+    }
+
+    pml4_dest = PHYS_TO_VIRT(paddr);
+    pml4_src = PHYS_TO_VIRT(vas.cr3);
+    for (uint16_t i = 0; i < 512; ++i) {
+        if (i < 256) {
+            pml4_dest[i] = 0;
+        } else {
+            pml4_dest[i] = pml4_src[i];
+        }
+    }
+
+    return 0;
+}
+
 void
 mu_pmap_init(void)
 {
diff --git a/sys/inc/mu/mmu.h b/sys/inc/mu/mmu.h
index ce52376..945e375 100644
--- a/sys/inc/mu/mmu.h
+++ b/sys/inc/mu/mmu.h
@@ -34,6 +34,12 @@
 #include <sys/cdefs.h>
 #include <md/vas.h>    /* shared */
 
+/*
+ * Copy the current VAS leaving the user-side
+ * zeroed
+ */
+__strong int mu_pmap_forkvas(struct mmu_vas *result);
+
 /*
  * Read the currently in-use virtual address space
  * structure
diff --git a/sys/mu/mmu_stub.c b/sys/mu/mmu_stub.c
index d2ab1eb..b53c3a4 100644
--- a/sys/mu/mmu_stub.c
+++ b/sys/mu/mmu_stub.c
@@ -30,6 +30,12 @@
 #include <sys/cdefs.h>
 #include <mu/mmu.h>
 
+__weak int
+mu_pmap_forkvas(struct mmu_vas *result)
+{
+    (void)result;
+}
+
 __weak void
 mu_pmap_init(void)
 {
-- 
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