Kernel: Implement byte ring buffer

This maps the ring twice right next to each other so we don't have to
care about wrapping around when doing memcpy or accessing the data

kernel/kernel/Memory/ByteRingBuffer.cpp

@@ -0,0 +1,49 @@
+#include <kernel/Memory/ByteRingBuffer.h>
+#include <kernel/Memory/Heap.h>
+#include <kernel/Memory/PageTable.h>
+
+namespace Kernel
+{
+
+	BAN::ErrorOr<BAN::UniqPtr<ByteRingBuffer>> ByteRingBuffer::create(size_t size)
+	{
+		ASSERT(size % PAGE_SIZE == 0);
+
+		const size_t page_count = size / PAGE_SIZE;
+
+		auto* buffer_ptr = new ByteRingBuffer(size);
+		if (buffer_ptr == nullptr)
+			return BAN::Error::from_errno(ENOMEM);
+		auto buffer = BAN::UniqPtr<ByteRingBuffer>::adopt(buffer_ptr);
+
+		buffer->m_vaddr = PageTable::kernel().reserve_free_contiguous_pages(page_count * 2, KERNEL_OFFSET);
+		if (buffer->m_vaddr == 0)
+			return BAN::Error::from_errno(ENOMEM);
+
+		for (size_t i = 0; i < page_count; i++)
+		{
+			const paddr_t paddr = Heap::get().take_free_page();
+			if (paddr == 0)
+				return BAN::Error::from_errno(ENOMEM);
+			PageTable::kernel().map_page_at(paddr, buffer->m_vaddr        + i * PAGE_SIZE, PageTable::ReadWrite | PageTable::Present);
+			PageTable::kernel().map_page_at(paddr, buffer->m_vaddr + size + i * PAGE_SIZE, PageTable::ReadWrite | PageTable::Present);
+		}
+
+		return buffer;
+	}
+
+	ByteRingBuffer::~ByteRingBuffer()
+	{
+		if (m_vaddr == 0)
+			return;
+		for (size_t i = 0; i < m_capacity / PAGE_SIZE; i++)
+		{
+			const paddr_t paddr = PageTable::kernel().physical_address_of(m_vaddr + i * PAGE_SIZE);
+			if (paddr == 0)
+				break;
+			Heap::get().release_page(paddr);
+		}
+		PageTable::kernel().unmap_range(m_vaddr, m_capacity * 2);
+	}
+
+}