Initial commit: Schunix as it stands before the SchOS rename

An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.

No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.

include/dma.h

@@ -0,0 +1,43 @@
+#pragma once
+
+// Physically contiguous memory for devices that read and write it themselves.
+//
+// kalloc() hands out 4 KB pages from a free list, in whatever order they were
+// released. That is exactly what a page table wants and exactly what a device
+// cannot use: an xHCI command ring, a NIC's descriptor array or a virtio
+// queue is a single physically contiguous object that the hardware walks
+// without any idea that paging exists. Two kalloc'd pages are adjacent in the
+// kernel's address space only by luck.
+//
+// So DMA memory comes from an arena carved out of the top of physical memory
+// before kalloc ever sees it, and is handed out contiguous and aligned.
+//
+// On x86 this memory needs no special cacheability. The PCI bus is cache
+// coherent: a device's write invalidates the corresponding cache line, and a
+// CPU write is snooped out of the store buffer before the device sees stale
+// data. Architectures without that guarantee need explicit cache maintenance
+// around every descriptor access, and porting this file is where that would
+// go. What is *not* free even here is ordering -- the compiler and the CPU
+// will happily reorder stores to a ring, so a driver still needs a barrier
+// between filling a descriptor and telling the device about it.
+
+#define DMA_ARENA_BYTES (4*1024*1024)
+
+void   dma_init(void *vstart, void *vend);
+
+// Zeroed, physically contiguous, at least `align`-aligned (align must be a
+// power of two). Returns a kernel virtual address, or 0.
+void  *dma_alloc(uint64 size, uint64 align);
+
+// `size` must be the size the allocation was made with, as in Linux's
+// dma_free_coherent(). Tracking it here would mean a second table whose only
+// job is to remember something the caller already knows.
+void   dma_free(void *p, uint64 size);
+
+// The address to give the device. Kernel memory is direct-mapped, so this is
+// just V2P -- but drivers should say what they mean, because on a machine
+// with an IOMMU these stop being the same number.
+addr_t dma_phys(const void *p);
+
+uint64 dma_free_bytes(void);
+uint64 dma_total_bytes(void);