[arch][x86] save the translated kernel pgdir physical address

This keeps from needing to recompute it on every context switch back to
the kernel aspace.

arch/include/arch/mmu.h

@@ -55,7 +55,8 @@ vaddr_t arch_mmu_pick_spot(arch_aspace_t *aspace,
                            vaddr_t end,  uint next_region_arch_mmu_flags,
                            vaddr_t align, size_t size, uint arch_mmu_flags) __NONNULL((1));
 
-/* load a new user address space context.
+/*
+ * load a new user address space context.
  * aspace argument NULL should unload user space.
  */
 void arch_mmu_context_switch(arch_aspace_t *aspace);

arch/x86/32/mmu.c

@@ -37,6 +37,7 @@
 map_addr_t kernel_pt[NO_OF_PT_ENTRIES][4] __ALIGNED(PAGE_SIZE);
 #endif
 map_addr_t kernel_pd[NO_OF_PT_ENTRIES] __ALIGNED(PAGE_SIZE);
+paddr_t kernel_pd_phys;
 
 static inline paddr_t get_pfn_from_pde(map_addr_t pde) {
     return pde & X86_4MB_PAGE_FRAME;
@@ -472,7 +473,7 @@ status_t arch_mmu_init_aspace(arch_aspace_t * const aspace, const vaddr_t base,
         aspace->base = base;
         aspace->size = size;
         aspace->cr3 = kernel_pd;
-        aspace->cr3_phys = vaddr_to_paddr(aspace->cr3);
+        aspace->cr3_phys = kernel_pd_phys;
     } else {
         DEBUG_ASSERT(base == USER_ASPACE_BASE);
         DEBUG_ASSERT(size == USER_ASPACE_SIZE);
@@ -499,6 +500,16 @@ status_t arch_mmu_init_aspace(arch_aspace_t * const aspace, const vaddr_t base,
 }
 
 status_t arch_mmu_destroy_aspace(arch_aspace_t * const aspace) {
+    // TODO: assert that we're not active on any cpus
+    if (aspace->flags & ARCH_ASPACE_FLAG_KERNEL) {
+        // can't destroy the kernel aspace
+        panic("attempt to destroy kernel aspace\n");
+        return ERR_NOT_ALLOWED;
+    }
+
+    // free the page table
+    pmm_free_kpages(aspace->cr3, 1);
+
     return NO_ERROR;
 }
 
@@ -512,8 +523,7 @@ void arch_mmu_context_switch(arch_aspace_t * const aspace) {
 
         cr3 = aspace->cr3_phys;
     } else {
-        // TODO save copy of this
-        cr3 = vaddr_to_paddr(kernel_pd);
+        cr3 = kernel_pd_phys;
     }
     if (TRACE_CONTEXT_SWITCH) {
         TRACEF("cr3 %#llx\n", cr3);

arch/x86/32/start.S

@@ -155,6 +155,9 @@ paging_setup:
     movl $PHYS(kernel_pd), %eax
     mov %eax, %cr3
 
+    /* save a copy of the address of the kernel page directory */
+    movl %eax, PHYS(kernel_pd_phys)
+
     /* Enabling Paging and from this point we are in */
     mov %cr0,  %eax
     btsl $(31), %eax

arch/x86/64/mmu.c

@@ -42,6 +42,9 @@ map_addr_t kernel_pml4[NO_OF_PT_ENTRIES] __ALIGNED(PAGE_SIZE);
 map_addr_t kernel_pdp[NO_OF_PT_ENTRIES] __ALIGNED(PAGE_SIZE); /* temporary */
 map_addr_t kernel_pte[NO_OF_PT_ENTRIES] __ALIGNED(PAGE_SIZE);
 
+/* saed physical address of the kernel_pml4 table */
+paddr_t kernel_pml4_phys;
+
 /* top level pdp needed to map the -512GB..0 space */
 map_addr_t kernel_pdp_high[NO_OF_PT_ENTRIES] __ALIGNED(PAGE_SIZE);
 
@@ -666,7 +669,7 @@ void x86_mmu_early_init(void) {
     kernel_pml4[0] = 0;
 
     /* tlb flush */
-    x86_set_cr3(x86_get_cr3());
+    x86_set_cr3(kernel_pml4_phys);
 }
 
 void x86_mmu_init(void) {
@@ -697,7 +700,7 @@ status_t arch_mmu_init_aspace(arch_aspace_t * const aspace, const vaddr_t base,
         aspace->base = base;
         aspace->size = size;
         aspace->cr3 = kernel_pml4;
-        aspace->cr3_phys = vaddr_to_paddr(aspace->cr3);
+        aspace->cr3_phys = kernel_pml4_phys;
     } else {
         DEBUG_ASSERT(base == USER_ASPACE_BASE);
         DEBUG_ASSERT(size == USER_ASPACE_SIZE);
@@ -737,18 +740,17 @@ status_t arch_mmu_destroy_aspace(arch_aspace_t *aspace) {
     return NO_ERROR;
 }
 
-void arch_mmu_context_switch(arch_aspace_t *aspace) {
+void arch_mmu_context_switch(arch_aspace_t *new_aspace) {
     if (TRACE_CONTEXT_SWITCH)
-        TRACEF("aspace %p\n", aspace);
+        TRACEF("aspace %p\n", new_aspace);
 
     uint64_t cr3;
-    if (aspace) {
-        DEBUG_ASSERT((aspace->flags & ARCH_ASPACE_FLAG_KERNEL) == 0);
+    if (new_aspace) {
+        DEBUG_ASSERT((new_aspace->flags & ARCH_ASPACE_FLAG_KERNEL) == 0);
 
-        cr3 = aspace->cr3_phys;
+        cr3 = new_aspace->cr3_phys;
     } else {
-        // TODO save copy of this
-        cr3 = vaddr_to_paddr(kernel_pml4);
+        cr3 = kernel_pml4_phys;
     }
     if (TRACE_CONTEXT_SWITCH) {
         TRACEF("cr3 %#llx\n", cr3);

arch/x86/64/start.S

@@ -103,6 +103,9 @@ paging_setup:
     movl $PHYS(kernel_pml4), %eax
     mov %eax, %cr3
 
+    /* save it into a global variable that is used by the kernel */
+    movl %eax, PHYS(kernel_pml4_phys)
+
     /* Long Mode Enabled at this point*/
     movl $MSR_EFER ,%ecx
     rdmsr