From b3ef38de81c03b5f86fd69805351d64760a1bf09 Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Mon, 26 Feb 2024 10:13:21 -0500
Subject: kernel/amd64: pmap: Add support for creating VAS

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 sys/arch/amd64/amd64/pmap.c | 39 +++++++++++++++++++++++++++++++++++++++
 sys/include/vm/pmap.h       |  4 ++++
 2 files changed, 43 insertions(+)

diff --git a/sys/arch/amd64/amd64/pmap.c b/sys/arch/amd64/amd64/pmap.c
index a952b41..8a21f63 100644
--- a/sys/arch/amd64/amd64/pmap.c
+++ b/sys/arch/amd64/amd64/pmap.c
@@ -29,9 +29,11 @@
 
 #include <vm/pmap.h>
 #include <vm/vm.h>
+#include <vm/physseg.h>
 #include <sys/cdefs.h>
 #include <machine/tlb.h>
 #include <assert.h>
+#include <string.h>
 
 /*
  * Page-Table Entry (PTE) flags
@@ -158,6 +160,43 @@ pmap_unmap(struct vm_ctx *ctx, vaddr_t va)
     return pmap_modify_tbl(ctx, va, 0);
 }
 
+struct vas
+pmap_create_vas(struct vm_ctx *ctx)
+{
+    struct vas current_vas = pmap_read_vas();
+    struct vas new_vas = {0};
+    uint64_t *src, *dest;
+
+    /*
+     * We want to allocate a zeroed pageframe
+     * and copy the higher half to it. The lower
+     * half can remain zero for userland.
+     */
+    new_vas.top_level = vm_alloc_pageframe(1);
+
+    if (new_vas.top_level == 0) {
+        /* Top level may remain zero to denote failure */
+        return new_vas;
+    }
+
+    src = PHYS_TO_VIRT(current_vas.top_level);
+    dest = PHYS_TO_VIRT(new_vas.top_level);
+
+    /*
+     * Copy the top half and zero the bottom
+     * half.
+     */
+    for (size_t i = 0; i < 512; ++i) {
+        if (i < 256) {
+            dest[i] = 0;
+            continue;
+        }
+        dest[i] = src[i];
+    }
+
+    return new_vas;
+}
+
 struct vas
 pmap_read_vas(void)
 {
diff --git a/sys/include/vm/pmap.h b/sys/include/vm/pmap.h
index fcef35d..6e37a00 100644
--- a/sys/include/vm/pmap.h
+++ b/sys/include/vm/pmap.h
@@ -53,6 +53,8 @@
 #define PMAP_WRITABLE __BIT(0)      /* Writable */
 #define PMAP_EXEC     __BIT(1)      /* Executable */
 
+#define is_vas_valid(vas) (vas.top_level != 0)
+
 /*
  * vm_ctx - Per core virtual memory context
  */
@@ -63,6 +65,8 @@ struct vm_ctx {
     struct spinlock dynalloc_lock;
 };
 
+struct vas pmap_create_vas(struct vm_ctx *);
+
 /*
  * Read virtual address space descriptor
  * and return it.
-- 
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