dm: block_if: support bypassing the Service VM's page cache

This patch adds an acrn-dm option `nocache` to bypass the Service VM's
page cache.
 - By default, the Service VM's page cache is utilized.
 - If `nocache` is specified in acrn-dm parameters, the Service VM's page cache
   would be bypassed (opening the file/block with O_DIRECT flag).

Example to bypass the Service VM's page cache:
`add_virtual_device    5 virtio-blk iothread,mq=2,/dev/nvme2n1,writeback,nocache`

Tracked-On: projectacrn#8612

Signed-off-by: Shiqing Gao <[email protected]>
Acked-by: Wang, Yu1 <[email protected]>

devicemodel/hw/block_if.c

@@ -829,6 +829,7 @@ blockif_open(const char *optstr, const char *ident, int queue_num, struct iothre
 	int max_discard_sectors, max_discard_seg, discard_sector_alignment;
 	off_t probe_arg[] = {0, 0};
 	int aio_mode;
+	int bypass_host_cache, open_flag;
 
 	pthread_once(&blockif_once, blockif_init);
 
@@ -850,6 +851,9 @@ blockif_open(const char *optstr, const char *ident, int queue_num, struct iothre
 	/* writethru is on by default */
 	writeback = 0;
 
+	/* By default, do NOT bypass Service VM's page cache. */
+	bypass_host_cache = 0;
+
 	candiscard = 0;
 
 	if (queue_num <= 0)
@@ -874,6 +878,8 @@ blockif_open(const char *optstr, const char *ident, int queue_num, struct iothre
 			writeback = 0;
 		else if (!strcmp(cp, "ro"))
 			ro = 1;
+		else if (!strcmp(cp, "nocache"))
+			bypass_host_cache = 1;
 		else if (!strncmp(cp, "discard", strlen("discard"))) {
 			strsep(&cp, "=");
 			if (cp != NULL) {
@@ -933,8 +939,12 @@ blockif_open(const char *optstr, const char *ident, int queue_num, struct iothre
 	 * after file is opened. Instead, we call fsync() after each write
 	 * operation to emulate it.
 	 */
+	open_flag = (ro ? O_RDONLY : O_RDWR);
+	if (bypass_host_cache == 1) {
+		open_flag |= O_DIRECT;
+	}
+	fd = open(nopt, open_flag);
 
-	fd = open(nopt, ro ? O_RDONLY : O_RDWR);
 	if (fd < 0 && !ro) {
 		/* Attempt a r/w fail with a r/o open */
 		fd = open(nopt, O_RDONLY);