commit d20373d3bfb8872cae43a8f2804e8a4d7aeb2e73
Author: Conrad Meyer <cem@FreeBSD.org>
Date:   Tue Oct 10 13:43:50 2017 -0700

    mem: Hack to allow libkvm to read device registers

diff --git a/sys/amd64/amd64/mem.c b/sys/amd64/amd64/mem.c
index 067da04dfc4f..9fb90767688f 100644
--- a/sys/amd64/amd64/mem.c
+++ b/sys/amd64/amd64/mem.c
@@ -73,6 +73,94 @@ __FBSDID("$FreeBSD$");
  */
 MALLOC_DEFINE(M_MEMDESC, "memdesc", "memory range descriptors");
 
+static bool
+is_word_aligned(u_int len, u_long addr)
+{
+	return ((len == 8 && (addr & 7) == 0) || (len == 4 && (addr & 3) == 0)
+	    || (len == 2 && (addr & 1) == 0));
+}
+
+static int
+alignmove(void *cp, u_int len, struct uio *uio)
+{
+	struct iovec *iov;
+	size_t cnt;
+	union {
+		uint64_t u64;
+		uint32_t u32;
+		uint16_t u16;
+	} devmem;
+	void *memp;
+	int error;
+
+	switch (len) {
+	case 8:
+		memp = &devmem.u64;
+		break;
+	case 4:
+		memp = &devmem.u32;
+		break;
+	case 2:
+		memp = &devmem.u16;
+		break;
+	default:
+		return (EINVAL);
+	}
+
+	error = 0;
+	iov = uio->uio_iov;
+	cnt = iov->iov_len;
+	if (cnt > len)
+		cnt = len;
+
+	if (uio->uio_rw == UIO_WRITE) {
+		if (uio->uio_segflg == UIO_USERSPACE) {
+			error = copyin(iov->iov_base, memp, cnt);
+			if (error != 0)
+				goto out;
+		} else if (uio->uio_segflg == UIO_SYSSPACE)
+			bcopy(iov->iov_base, memp, cnt);
+
+		switch (len) {
+		case 8:
+			*(volatile uint64_t *)cp = devmem.u64;
+			break;
+		case 4:
+			*(volatile uint32_t *)cp = devmem.u32;
+			break;
+		case 2:
+			*(volatile uint16_t *)cp = devmem.u16;
+			break;
+		}
+	} else {
+		switch (len) {
+		case 8:
+			devmem.u64 = *(volatile uint64_t *)cp;
+			break;
+		case 4:
+			devmem.u32 = *(volatile uint32_t *)cp;
+			break;
+		case 2:
+			devmem.u16 = *(volatile uint16_t *)cp;
+			break;
+		}
+
+		if (uio->uio_segflg == UIO_USERSPACE) {
+			error = copyout(memp, iov->iov_base, cnt);
+			if (error != 0)
+				goto out;
+		} else if (uio->uio_segflg == UIO_SYSSPACE)
+			bcopy(memp, iov->iov_base, cnt);
+	}
+
+	iov->iov_base = (char *)iov->iov_base + cnt;
+	iov->iov_len -= cnt;
+	uio->uio_resid -= cnt;
+	uio->uio_offset += cnt;
+out:
+	return (error);
+}
+
 /* ARGSUSED */
 int
 memrw(struct cdev *dev, struct uio *uio, int flags)
@@ -107,7 +195,10 @@ memrw(struct cdev *dev, struct uio *uio, int flags)
 			 */
 			if (v >= DMAP_MIN_ADDRESS &&
 			    v < DMAP_MIN_ADDRESS + dmaplimit) {
-				error = uiomove((void *)v, c, uio);
+				if (is_word_aligned(c, v))
+					error = alignmove((void *)v, c, uio);
+				else
+					error = uiomove((void *)v, c, uio);
 				break;
 			}
 
@@ -137,7 +228,10 @@ memrw(struct cdev *dev, struct uio *uio, int flags)
 		case CDEV_MINOR_MEM:
 			if (v < dmaplimit) {
 				vd = PHYS_TO_DMAP(v);
-				error = uiomove((void *)vd, c, uio);
+				if (is_word_aligned(c, vd))
+					error = alignmove((void *)vd, c, uio);
+				else
+					error = uiomove((void *)vd, c, uio);
 				break;
 			}
 			if (v > cpu_getmaxphyaddr()) {
@@ -145,7 +239,10 @@ memrw(struct cdev *dev, struct uio *uio, int flags)
 				break;
 			}
 			p = pmap_mapdev(v, PAGE_SIZE);
-			error = uiomove(p, c, uio);
+			if (is_word_aligned(c, (u_long)p))
+				error = alignmove(p, c, uio);
+			else
+				error = uiomove(p, c, uio);
 			pmap_unmapdev((vm_offset_t)p, PAGE_SIZE);
 			break;
 		}