[kernel] Rename stack_cache to buffer_cache

Renamed and genericized the stack_cache class to manage any address
range area of buffers or memory regions. Removed singleton and created
some globals that map to different address regions (kernel stacks,
kernel buffers).

Tags: vmem virtual memeory

src/kernel/buffer_cache.cpp

@@ -0,0 +1,48 @@
+#include "kutil/assert.h"
+#include "kutil/vm_space.h"
+#include "kernel_memory.h"
+#include "page_manager.h"
+#include "buffer_cache.h"
+
+extern kutil::vm_space g_kernel_space;
+
+using memory::frame_size;
+using memory::kernel_stack_pages;
+using memory::kernel_buffer_pages;
+
+static constexpr size_t stack_bytes = kernel_stack_pages * frame_size;
+static constexpr size_t buffer_bytes = kernel_buffer_pages * frame_size;
+
+buffer_cache g_kstack_cache {memory::stacks_start, stack_bytes, memory::kernel_max_stacks};
+buffer_cache g_kbuffer_cache {memory::buffers_start, buffer_bytes, memory::kernel_max_buffers};
+
+buffer_cache::buffer_cache(uintptr_t start, size_t size, size_t length) :
+	m_next(start), m_end(start+length), m_size(size)
+{
+	kassert((size % frame_size) == 0, "buffer_cache given a non-page-multiple size!");
+}
+
+uintptr_t
+buffer_cache::get_buffer()
+{
+	uintptr_t addr = 0;
+	if (m_cache.count() > 0) {
+		addr = m_cache.pop();
+	} else {
+		addr = m_next;
+		m_next += m_size;
+	}
+
+	g_kernel_space.commit(addr, m_size);
+	return addr;
+}
+
+void
+buffer_cache::return_buffer(uintptr_t addr)
+{
+	void *ptr = reinterpret_cast<void*>(addr);
+	size_t page_count = page_manager::page_count(m_size);
+	page_manager::get()->unmap_pages(ptr, page_count);
+	g_kernel_space.unreserve(addr, m_size);
+	m_cache.append(addr);
+}