[PATCH 2/3] scatterlist: add sgl_equal_sgl() function

[Douglas Gilbert]

After enabling copies between scatter gather lists (sgl_s), another
storage related operation is to compare two sgl_s for equality. This
new function is designed to partially implement NVMe's Compare
command and the SCSI VERIFY(BYTCHK=1) command. Like memcmp() this
function begins scanning at the start (of each sgl) and returns
false on the first miscompare and stops comparing.

The sgl_equal_sgl_idx() function additionally yields the index (i.e.
byte position) of the first miscompare. The additional parameter,
miscompare_idx, is a pointer. If it is non-NULL and a miscompare is
detected (i.e. the function returns false) then the byte index of
the first miscompare is written to *miscompare_idx. Knowing the
location of the first miscompare is needed to implement properly
the SCSI COMPARE AND WRITE command.

Reviewed-by: Bodo Stroesser <bostroesser@xxxxxxxxx>
Signed-off-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>
---
 include/linux/scatterlist.h |   8 +++
 lib/scatterlist.c           | 110 ++++++++++++++++++++++++++++++++++++
 2 files changed, 118 insertions(+)

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 22111ee21383..40449ce96a18 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -324,6 +324,14 @@ size_t sgl_copy_sgl(struct scatterlist *d_sgl, unsigned int d_nents, off_t d_ski
 		    struct scatterlist *s_sgl, unsigned int s_nents, off_t s_skip,
 		    size_t n_bytes);
 
+bool sgl_equal_sgl(struct scatterlist *x_sgl, unsigned int x_nents, off_t x_skip,
+		   struct scatterlist *y_sgl, unsigned int y_nents, off_t y_skip,
+		   size_t n_bytes);
+
+bool sgl_equal_sgl_idx(struct scatterlist *x_sgl, unsigned int x_nents, off_t x_skip,
+		       struct scatterlist *y_sgl, unsigned int y_nents, off_t y_skip,
+		       size_t n_bytes, size_t *miscompare_idx);
+
 /*
  * Maximum number of entries that will be allocated in one piece, if
  * a list larger than this is required then chaining will be utilized.
diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index 782bcfe72c60..a8672bc6d883 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -1132,3 +1132,113 @@ size_t sgl_copy_sgl(struct scatterlist *d_sgl, unsigned int d_nents, off_t d_ski
 	return offset;
 }
 EXPORT_SYMBOL(sgl_copy_sgl);
+
+/**
+ * sgl_equal_sgl_idx - check if x and y (both sgl_s) compare equal, report
+ *		       index for first unequal bytes
+ * @x_sgl:		 x (left) sgl
+ * @x_nents:		 Number of SG entries in x (left) sgl
+ * @x_skip:		 Number of bytes to skip in x (left) before starting
+ * @y_sgl:		 y (right) sgl
+ * @y_nents:		 Number of SG entries in y (right) sgl
+ * @y_skip:		 Number of bytes to skip in y (right) before starting
+ * @n_bytes:		 The (maximum) number of bytes to compare
+ * @miscompare_idx:	 if return is false, index of first miscompare written
+ *			 to this pointer (if non-NULL). Value will be < n_bytes
+ *
+ * Returns:
+ *   true if x and y compare equal before x, y or n_bytes is exhausted.
+ *   Otherwise on a miscompare, returns false (and stops comparing). If return
+ *   is false and miscompare_idx is non-NULL, then index of first miscompared
+ *   byte written to *miscompare_idx.
+ *
+ * Notes:
+ *   x and y are symmetrical: they can be swapped and the result is the same.
+ *
+ *   Implementation is based on memcmp(). x and y segments may overlap.
+ *
+ *   The notes in sgl_copy_sgl() about large sgl_s _applies here as well.
+ *
+ **/
+bool sgl_equal_sgl_idx(struct scatterlist *x_sgl, unsigned int x_nents, off_t x_skip,
+		       struct scatterlist *y_sgl, unsigned int y_nents, off_t y_skip,
+		       size_t n_bytes, size_t *miscompare_idx)
+{
+	bool equ = true;
+	size_t len;
+	size_t offset = 0;
+	struct sg_mapping_iter x_iter, y_iter;
+
+	if (n_bytes == 0)
+		return true;
+	sg_miter_start(&x_iter, x_sgl, x_nents, SG_MITER_ATOMIC | SG_MITER_FROM_SG);
+	sg_miter_start(&y_iter, y_sgl, y_nents, SG_MITER_ATOMIC | SG_MITER_FROM_SG);
+	if (!sg_miter_skip(&x_iter, x_skip))
+		goto fini;
+	if (!sg_miter_skip(&y_iter, y_skip))
+		goto fini;
+
+	while (offset < n_bytes) {
+		if (!sg_miter_next(&x_iter))
+			break;
+		if (!sg_miter_next(&y_iter))
+			break;
+		len = min3(x_iter.length, y_iter.length, n_bytes - offset);
+
+		equ = !memcmp(x_iter.addr, y_iter.addr, len);
+		if (!equ)
+			goto fini;
+		offset += len;
+		/* LIFO order is important when SG_MITER_ATOMIC is used */
+		y_iter.consumed = len;
+		sg_miter_stop(&y_iter);
+		x_iter.consumed = len;
+		sg_miter_stop(&x_iter);
+	}
+fini:
+	if (miscompare_idx && !equ) {
+		u8 *xp = x_iter.addr;
+		u8 *yp = y_iter.addr;
+		u8 *x_endp;
+
+		for (x_endp = xp + len ; xp < x_endp; ++xp, ++yp) {
+			if (*xp != *yp)
+				break;
+		}
+		*miscompare_idx = offset + len - (x_endp - xp);
+	}
+	sg_miter_stop(&y_iter);
+	sg_miter_stop(&x_iter);
+	return equ;
+}
+EXPORT_SYMBOL(sgl_equal_sgl_idx);
+
+/**
+ * sgl_equal_sgl - check if x and y (both sgl_s) compare equal
+ * @x_sgl:		 x (left) sgl
+ * @x_nents:		 Number of SG entries in x (left) sgl
+ * @x_skip:		 Number of bytes to skip in x (left) before starting
+ * @y_sgl:		 y (right) sgl
+ * @y_nents:		 Number of SG entries in y (right) sgl
+ * @y_skip:		 Number of bytes to skip in y (right) before starting
+ * @n_bytes:		 The (maximum) number of bytes to compare
+ *
+ * Returns:
+ *   true if x and y compare equal before x, y or n_bytes is exhausted.
+ *   Otherwise on a miscompare, returns false (and stops comparing).
+ *
+ * Notes:
+ *   x and y are symmetrical: they can be swapped and the result is the same.
+ *
+ *   Implementation is based on memcmp(). x and y segments may overlap.
+ *
+ *   The notes in sgl_copy_sgl() about large sgl_s _applies here as well.
+ *
+ **/
+bool sgl_equal_sgl(struct scatterlist *x_sgl, unsigned int x_nents, off_t x_skip,
+		   struct scatterlist *y_sgl, unsigned int y_nents, off_t y_skip,
+		   size_t n_bytes)
+{
+	return sgl_equal_sgl_idx(x_sgl, x_nents, x_skip, y_sgl, y_nents, y_skip, n_bytes, NULL);
+}
+EXPORT_SYMBOL(sgl_equal_sgl);
-- 
2.25.1