[kernel] Add a _very_ basic TLB shootdown mechanism

This TLB shootdown implementation is pretty janky: When the kernel
unmaps a mapping, it sends an IPI to all other cores and doesn't even
wait to ensure they've finished handling it. Upon getting one of these
IPIs, the core just re-writes cr3 to flush all TLBs.

src/kernel/interrupt_isrs.inc

@@ -117,4 +117,5 @@ ISR (0xe1, 0, isrLINT0)
 ISR (0xe2, 0, isrLINT1)
 ISR (0xe3, 0, isrAPICError)
 ISR (0xe4, 0, ipiSchedule)
+ISR (0xe5, 0, ipiShootdown)
 

src/kernel/interrupts.cpp

@@ -21,6 +21,7 @@ constexpr uintptr_t apic_eoi_addr = 0xfee000b0 + mem::linear_offset;
 extern "C" {
     void isr_handler(cpu_state*);
     void irq_handler(cpu_state*);
+    void _reload_cr3();
 }
 
 uint8_t
@@ -187,6 +188,11 @@ isr_handler(cpu_state *regs)
         scheduler::get().schedule();
         break;
 
+    case isr::ipiShootdown:
+        // TODO: Real shootdown algorithm
+        _reload_cr3();
+        break;
+
     default:
         util::format({message, sizeof(message)}, "Unknown interrupt 0x%lx", regs->interrupt);
         kassert(false, message, regs);

src/kernel/task.s

@@ -91,3 +91,10 @@ _current_gsbase:
 	mov rax, [gs:CPU_DATA.self]
 	ret
 .end:
+
+global _reload_cr3: function hidden (_reload_cr3.end - _reload_cr3)
+_reload_cr3:
+    mov rax, cr3
+    mov cr3, rax
+    ret
+.end:

src/kernel/vm_space.cpp

@@ -1,6 +1,7 @@
 #include <j6/memutils.h>
 #include <arch/memory.h>
 
+#include "apic.h"
 #include "kassert.h"
 #include "frame_allocator.h"
 #include "logger.h"
@@ -81,7 +82,7 @@ vm_space::kernel_space()
 }
 
 uintptr_t
-vm_space::add(uintptr_t base, obj::vm_area *area, util::bitset32 flags)
+vm_space::add(uintptr_t base, obj::vm_area *new_area, util::bitset32 flags)
 {
     if (!base)
         base = min_auto_address;
@@ -89,23 +90,23 @@ vm_space::add(uintptr_t base, obj::vm_area *area, util::bitset32 flags)
     //TODO: optimize find/insert
     bool exact = flags.get(vm_flags::exact);
     for (size_t i = 0; i < m_areas.count(); ++i) {
-        const vm_space::area &a = m_areas[i];
+        const vm_space::area &cur = m_areas[i];
-        uintptr_t aend = a.base + a.area->size();
+        uintptr_t cur_end = cur.base + cur.area->size();
-        if (base >= aend)
+        if (base >= cur_end)
             continue;
 
-        uintptr_t end = base + area->size();
+        uintptr_t end = base + new_area->size();
-        if (end <= a.base)
+        if (end <= cur.base)
             break;
         else if (exact)
             return 0;
         else
-            base = aend;
+            base = cur_end;
     }
 
-    m_areas.sorted_insert({base, area});
+    m_areas.sorted_insert({base, new_area});
-    area->add_to(this);
+    new_area->add_to(this);
-    area->handle_retain();
+    new_area->handle_retain();
     return base;
 }
 
@@ -267,6 +268,9 @@ vm_space::clear(const obj::vm_area &vma, uintptr_t offset, size_t count, bool fr
         ++it;
     }
 
+    current_cpu().apic->send_ipi_broadcast(
+        lapic::ipi_fixed, false, isr::ipiShootdown);
+
     if (free && free_count)
         fa.free(free_start, free_count);
 }