diff --git a/sys/conf/files b/sys/conf/files
--- a/sys/conf/files
+++ b/sys/conf/files
@@ -3879,6 +3879,7 @@
 kern/subr_lock.c		standard
 kern/subr_log.c			standard
 kern/subr_mchain.c		optional libmchain
+kern/subr_memdesc.c		standard
 kern/subr_module.c		standard
 kern/subr_msan.c		optional kmsan \
 	compile-with "${NORMAL_C:N-fsanitize*:N-fstack-protector*}"
diff --git a/sys/kern/subr_memdesc.c b/sys/kern/subr_memdesc.c
new file mode 100644
--- /dev/null
+++ b/sys/kern/subr_memdesc.c
@@ -0,0 +1,325 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2023 Chelsio Communications, Inc.
+ * Written by: John Baldwin <jhb@FreeBSD.org>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/param.h>
+#include <sys/mbuf.h>
+#include <sys/memdesc.h>
+#include <sys/systm.h>
+#include <sys/uio.h>
+#include <vm/vm.h>
+#include <vm/pmap.h>
+#include <vm/vm_param.h>
+#include <machine/bus.h>
+
+static void
+phys_copyback(vm_paddr_t pa, int off, int size, const void *src)
+{
+	const char *cp;
+	u_int page_off;
+	int todo;
+	void *p;
+
+	KASSERT(PMAP_HAS_DMAP, ("direct-map required"));
+
+	cp = src;
+	pa += off;
+	page_off = pa & PAGE_MASK;
+	while (size > 0) {
+		todo = min(PAGE_SIZE - page_off, size);
+		p = (void *)PHYS_TO_DMAP(pa);
+		memcpy(p, cp, todo);
+		size -= todo;
+		cp += todo;
+		pa += todo;
+		page_off = 0;
+	}
+}
+
+static void
+vlist_copyback(struct bus_dma_segment *vlist, int sglist_cnt, int off,
+    int size, const void *src)
+{
+	const char *p;
+	int todo;
+
+	while (vlist->ds_len <= off) {
+		KASSERT(sglist_cnt > 1, ("out of sglist entries"));
+
+		off -= vlist->ds_len;
+		vlist++;
+		sglist_cnt--;
+	}
+
+	p = src;
+	while (size > 0) {
+		KASSERT(sglist_cnt >= 1, ("out of sglist entries"));
+
+		todo = size;
+		if (todo > vlist->ds_len - off)
+			todo = vlist->ds_len - off;
+
+		memcpy((char *)(uintptr_t)vlist->ds_addr + off, p, todo);
+		off = 0;
+		vlist++;
+		sglist_cnt--;
+		size -= todo;
+		p += todo;
+	}
+}
+
+static void
+plist_copyback(struct bus_dma_segment *plist, int sglist_cnt, int off,
+    int size, const void *src)
+{
+	const char *p;
+	int todo;
+
+	while (plist->ds_len <= off) {
+		KASSERT(sglist_cnt > 1, ("out of sglist entries"));
+
+		off -= plist->ds_len;
+		plist++;
+		sglist_cnt--;
+	}
+
+	p = src;
+	while (size > 0) {
+		KASSERT(sglist_cnt >= 1, ("out of sglist entries"));
+
+		todo = size;
+		if (todo > plist->ds_len - off)
+			todo = plist->ds_len - off;
+
+		phys_copyback(plist->ds_addr, off, todo, p);
+		off = 0;
+		plist++;
+		sglist_cnt--;
+		size -= todo;
+		p += todo;
+	}
+}
+
+static void
+vmpages_copyback(vm_page_t *m, int off, int size, const void *src)
+{
+	struct iovec iov[1];
+	struct uio uio;
+	int error __diagused;
+
+	iov[0].iov_base = __DECONST(void *, src);
+	iov[0].iov_len = size;
+	uio.uio_iov = iov;
+	uio.uio_iovcnt = 1;
+	uio.uio_offset = 0;
+	uio.uio_resid = size;
+	uio.uio_segflg = UIO_SYSSPACE;
+	uio.uio_rw = UIO_WRITE;
+	error = uiomove_fromphys(m, off, size, &uio);
+	KASSERT(error == 0 && uio.uio_resid == 0, ("copy failed"));
+}
+
+void
+memdesc_copyback(struct memdesc *mem, int off, int size, const void *src)
+{
+	KASSERT(off >= 0, ("%s: invalid offset %d", __func__, off));
+	KASSERT(size >= 0, ("%s: invalid size %d", __func__, off));
+
+	switch (mem->md_type) {
+	case MEMDESC_VADDR:
+		KASSERT(off + size <= mem->md_len, ("copy out of bounds"));
+		memcpy((char *)mem->u.md_vaddr + off, src, size);
+		break;
+	case MEMDESC_PADDR:
+		KASSERT(off + size <= mem->md_len, ("copy out of bounds"));
+		phys_copyback(mem->u.md_paddr, off, size, src);
+		break;
+	case MEMDESC_VLIST:
+		vlist_copyback(mem->u.md_list, mem->md_nseg, off, size, src);
+		break;
+	case MEMDESC_PLIST:
+		plist_copyback(mem->u.md_list, mem->md_nseg, off, size, src);
+		break;
+	case MEMDESC_UIO:
+		panic("Use uiomove instead");
+		break;
+	case MEMDESC_MBUF:
+		m_copyback(mem->u.md_mbuf, off, size, src);
+		break;
+	case MEMDESC_VMPAGES:
+		KASSERT(off + size <= mem->md_len, ("copy out of bounds"));
+		vmpages_copyback(mem->u.md_ma, mem->md_offset + off, size,
+		    src);
+		break;
+	default:
+		__assert_unreachable();
+	}
+}
+
+static void
+phys_copydata(vm_paddr_t pa, int off, int size, void *dst)
+{
+	char *cp;
+	u_int page_off;
+	int todo;
+	const void *p;
+
+	KASSERT(PMAP_HAS_DMAP, ("direct-map required"));
+
+	cp = dst;
+	pa += off;
+	page_off = pa & PAGE_MASK;
+	while (size > 0) {
+		todo = min(PAGE_SIZE - page_off, size);
+		p = (const void *)PHYS_TO_DMAP(pa);
+		memcpy(cp, p, todo);
+		size -= todo;
+		cp += todo;
+		pa += todo;
+		page_off = 0;
+	}
+}
+
+static void
+vlist_copydata(struct bus_dma_segment *vlist, int sglist_cnt, int off,
+    int size, void *dst)
+{
+	char *p;
+	int todo;
+
+	while (vlist->ds_len <= off) {
+		KASSERT(sglist_cnt > 1, ("out of sglist entries"));
+
+		off -= vlist->ds_len;
+		vlist++;
+		sglist_cnt--;
+	}
+
+	p = dst;
+	while (size > 0) {
+		KASSERT(sglist_cnt >= 1, ("out of sglist entries"));
+
+		todo = size;
+		if (todo > vlist->ds_len - off)
+			todo = vlist->ds_len - off;
+
+		memcpy(p, (char *)(uintptr_t)vlist->ds_addr + off, todo);
+		off = 0;
+		vlist++;
+		sglist_cnt--;
+		size -= todo;
+		p += todo;
+	}
+}
+
+static void
+plist_copydata(struct bus_dma_segment *plist, int sglist_cnt, int off,
+    int size, void *dst)
+{
+	char *p;
+	int todo;
+
+	while (plist->ds_len <= off) {
+		KASSERT(sglist_cnt > 1, ("out of sglist entries"));
+
+		off -= plist->ds_len;
+		plist++;
+		sglist_cnt--;
+	}
+
+	p = dst;
+	while (size > 0) {
+		KASSERT(sglist_cnt >= 1, ("out of sglist entries"));
+
+		todo = size;
+		if (todo > plist->ds_len - off)
+			todo = plist->ds_len - off;
+
+		phys_copydata(plist->ds_addr, off, todo, p);
+		off = 0;
+		plist++;
+		sglist_cnt--;
+		size -= todo;
+		p += todo;
+	}
+}
+
+static void
+vmpages_copydata(vm_page_t *m, int off, int size, void *dst)
+{
+	struct iovec iov[1];
+	struct uio uio;
+	int error __diagused;
+
+	iov[0].iov_base = dst;
+	iov[0].iov_len = size;
+	uio.uio_iov = iov;
+	uio.uio_iovcnt = 1;
+	uio.uio_offset = 0;
+	uio.uio_resid = size;
+	uio.uio_segflg = UIO_SYSSPACE;
+	uio.uio_rw = UIO_READ;
+	error = uiomove_fromphys(m, off, size, &uio);
+	KASSERT(error == 0 && uio.uio_resid == 0, ("copy failed"));
+}
+
+void
+memdesc_copydata(struct memdesc *mem, int off, int size, void *dst)
+{
+	KASSERT(off >= 0, ("%s: invalid offset %d", __func__, off));
+	KASSERT(size >= 0, ("%s: invalid size %d", __func__, off));
+
+	switch (mem->md_type) {
+	case MEMDESC_VADDR:
+		KASSERT(off + size <= mem->md_len, ("copy out of bounds"));
+		memcpy(dst, (const char *)mem->u.md_vaddr + off, size);
+		break;
+	case MEMDESC_PADDR:
+		KASSERT(off + size <= mem->md_len, ("copy out of bounds"));
+		phys_copydata(mem->u.md_paddr, off, size, dst);
+		break;
+	case MEMDESC_VLIST:
+		vlist_copydata(mem->u.md_list, mem->md_nseg, off, size, dst);
+		break;
+	case MEMDESC_PLIST:
+		plist_copydata(mem->u.md_list, mem->md_nseg, off, size, dst);
+		break;
+	case MEMDESC_UIO:
+		panic("Use uiomove instead");
+		break;
+	case MEMDESC_MBUF:
+		m_copydata(mem->u.md_mbuf, off, size, dst);
+		break;
+	case MEMDESC_VMPAGES:
+		KASSERT(off + size <= mem->md_len, ("copy out of bounds"));
+		vmpages_copydata(mem->u.md_ma, mem->md_offset + off, size,
+		    dst);
+		break;
+	default:
+		__assert_unreachable();
+	}
+}
diff --git a/sys/sys/memdesc.h b/sys/sys/memdesc.h
--- a/sys/sys/memdesc.h
+++ b/sys/sys/memdesc.h
@@ -155,4 +155,14 @@
 struct memdesc	memdesc_bio(struct bio *bio);
 struct memdesc	memdesc_ccb(union ccb *ccb);
 
+/*
+ * Similar to m_copyback/data, *_copyback copy data from the 'src'
+ * buffer into the memory descriptor's data buffer while *_copydata
+ * copy data from the memory descriptor's data buffer into the the
+ * 'dst' buffer.
+ */
+void	memdesc_copyback(struct memdesc *mem, int off, int size,
+    const void *src);
+void	memdesc_copydata(struct memdesc *mem, int off, int size, void *dst);
+
 #endif /* _SYS_MEMDESC_H_ */