Index: head/sys/dev/sfxge/common/ef10_impl.h
===================================================================
--- head/sys/dev/sfxge/common/ef10_impl.h
+++ head/sys/dev/sfxge/common/ef10_impl.h
@@ -445,6 +445,14 @@
 	__in			size_t size);
 
 extern	__checkReturn		efx_rc_t
+ef10_nvram_partn_read_backup(
+	__in			efx_nic_t *enp,
+	__in			uint32_t partn,
+	__in			unsigned int offset,
+	__out_bcount(size)	caddr_t data,
+	__in			size_t size);
+
+extern	__checkReturn		efx_rc_t
 ef10_nvram_partn_erase(
 	__in			efx_nic_t *enp,
 	__in			uint32_t partn,
Index: head/sys/dev/sfxge/common/ef10_nvram.c
===================================================================
--- head/sys/dev/sfxge/common/ef10_nvram.c
+++ head/sys/dev/sfxge/common/ef10_nvram.c
@@ -1940,11 +1940,34 @@
 	__in			size_t size)
 {
 	/*
-	 * Read requests which come in through the EFX API expect to
-	 * read the current, active partition.
+	 * An A/B partition has two data stores (current and backup).
+	 * Read requests which come in through the EFX API expect to read the
+	 * current, active store of an A/B partition. For non A/B partitions,
+	 * there is only a single store and so the mode param is ignored.
 	 */
 	return ef10_nvram_partn_read_mode(enp, partn, offset, data, size,
 			    MC_CMD_NVRAM_READ_IN_V2_TARGET_CURRENT);
+}
+
+	__checkReturn		efx_rc_t
+ef10_nvram_partn_read_backup(
+	__in			efx_nic_t *enp,
+	__in			uint32_t partn,
+	__in			unsigned int offset,
+	__out_bcount(size)	caddr_t data,
+	__in			size_t size)
+{
+	/*
+	 * An A/B partition has two data stores (current and backup).
+	 * Read the backup store of an A/B partition (i.e. the store currently
+	 * being written to if the partition is locked).
+	 *
+	 * This is needed when comparing the existing partition content to avoid
+	 * unnecessary writes, or to read back what has been written to check
+	 * that the writes have succeeded.
+	 */
+	return ef10_nvram_partn_read_mode(enp, partn, offset, data, size,
+			    MC_CMD_NVRAM_READ_IN_V2_TARGET_BACKUP);
 }
 
 	__checkReturn		efx_rc_t
Index: head/sys/dev/sfxge/common/efx.h
===================================================================
--- head/sys/dev/sfxge/common/efx.h
+++ head/sys/dev/sfxge/common/efx.h
@@ -1437,6 +1437,14 @@
 	__in			size_t size);
 
 extern	__checkReturn		efx_rc_t
+efx_nvram_read_backup(
+	__in			efx_nic_t *enp,
+	__in			efx_nvram_type_t type,
+	__in			unsigned int offset,
+	__out_bcount(size)	caddr_t data,
+	__in			size_t size);
+
+extern	__checkReturn		efx_rc_t
 efx_nvram_set_version(
 	__in			efx_nic_t *enp,
 	__in			efx_nvram_type_t type,
Index: head/sys/dev/sfxge/common/efx_impl.h
===================================================================
--- head/sys/dev/sfxge/common/efx_impl.h
+++ head/sys/dev/sfxge/common/efx_impl.h
@@ -471,6 +471,8 @@
 	efx_rc_t	(*envo_partn_rw_start)(efx_nic_t *, uint32_t, size_t *);
 	efx_rc_t	(*envo_partn_read)(efx_nic_t *, uint32_t,
 					    unsigned int, caddr_t, size_t);
+	efx_rc_t	(*envo_partn_read_backup)(efx_nic_t *, uint32_t,
+					    unsigned int, caddr_t, size_t);
 	efx_rc_t	(*envo_partn_erase)(efx_nic_t *, uint32_t,
 					    unsigned int, size_t);
 	efx_rc_t	(*envo_partn_write)(efx_nic_t *, uint32_t,
Index: head/sys/dev/sfxge/common/efx_nvram.c
===================================================================
--- head/sys/dev/sfxge/common/efx_nvram.c
+++ head/sys/dev/sfxge/common/efx_nvram.c
@@ -48,6 +48,7 @@
 	siena_nvram_partn_size,		/* envo_partn_size */
 	siena_nvram_partn_rw_start,	/* envo_partn_rw_start */
 	siena_nvram_partn_read,		/* envo_partn_read */
+	siena_nvram_partn_read,		/* envo_partn_read_backup */
 	siena_nvram_partn_erase,	/* envo_partn_erase */
 	siena_nvram_partn_write,	/* envo_partn_write */
 	siena_nvram_partn_rw_finish,	/* envo_partn_rw_finish */
@@ -68,6 +69,7 @@
 	ef10_nvram_partn_size,		/* envo_partn_size */
 	ef10_nvram_partn_rw_start,	/* envo_partn_rw_start */
 	ef10_nvram_partn_read,		/* envo_partn_read */
+	ef10_nvram_partn_read_backup,	/* envo_partn_read_backup */
 	ef10_nvram_partn_erase,		/* envo_partn_erase */
 	ef10_nvram_partn_write,		/* envo_partn_write */
 	ef10_nvram_partn_rw_finish,	/* envo_partn_rw_finish */
@@ -278,6 +280,48 @@
 	EFSYS_ASSERT3U(enp->en_nvram_partn_locked, ==, partn);
 
 	if ((rc = envop->envo_partn_read(enp, partn, offset, data, size)) != 0)
+		goto fail2;
+
+	return (0);
+
+fail2:
+	EFSYS_PROBE(fail2);
+fail1:
+	EFSYS_PROBE1(fail1, efx_rc_t, rc);
+
+	return (rc);
+}
+
+/*
+ * Read from the backup (writeable) store of an A/B partition.
+ * For non A/B partitions, there is only a single store, and so this
+ * function has the same behaviour as efx_nvram_read_chunk().
+ */
+	__checkReturn		efx_rc_t
+efx_nvram_read_backup(
+	__in			efx_nic_t *enp,
+	__in			efx_nvram_type_t type,
+	__in			unsigned int offset,
+	__out_bcount(size)	caddr_t data,
+	__in			size_t size)
+{
+	const efx_nvram_ops_t *envop = enp->en_envop;
+	uint32_t partn;
+	efx_rc_t rc;
+
+	EFSYS_ASSERT3U(enp->en_magic, ==, EFX_NIC_MAGIC);
+	EFSYS_ASSERT3U(enp->en_mod_flags, &, EFX_MOD_NVRAM);
+
+	EFSYS_ASSERT3U(type, <, EFX_NVRAM_NTYPES);
+	EFSYS_ASSERT3U(type, !=, EFX_NVRAM_INVALID);
+
+	if ((rc = envop->envo_type_to_partn(enp, type, &partn)) != 0)
+		goto fail1;
+
+	EFSYS_ASSERT3U(enp->en_nvram_partn_locked, ==, partn);
+
+	if ((rc = envop->envo_partn_read_backup(enp, partn, offset,
+		    data, size)) != 0)
 		goto fail2;
 
 	return (0);