block: add enable_write_cache flag

Add a enable_write_cache flag in the block driver state, and use it to
decide if we claim to have a volatile write cache that needs controlled
flushing from the guest.  The flag is off if cache=writethrough is
defined because O_DSYNC guarantees that every write goes to stable
storage, and it is on for cache=none and cache=writeback.

Both scsi-disk and ide now use the new flage, changing from their
defaults of always off (ide) or always on (scsi-disk).

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

5 files changed, 27 insertions, 3 deletions

diff --git a/block.c b/block.c
index 45ad6fb4b9..da0930cbba 100644
--- a/block.c
+++ b/block.c
@@ -408,6 +408,16 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
     }
     bs->drv = drv;
     bs->opaque = qemu_mallocz(drv->instance_size);
+
+    /*
+     * Yes, BDRV_O_NOCACHE aka O_DIRECT means we have to present a
+     * write cache to the guest.  We do need the fdatasync to flush
+     * out transactions for block allocations, and we maybe have a
+     * volatile write cache in our backing device to deal with.
+     */
+    if (flags & (BDRV_O_CACHE_WB|BDRV_O_NOCACHE))
+        bs->enable_write_cache = 1;
+
     /* Note: for compatibility, we open disk image files as RDWR, and
        RDONLY as fallback */
     if (!(flags & BDRV_O_FILE))
@@ -918,6 +928,11 @@ int bdrv_is_sg(BlockDriverState *bs)
     return bs->sg;
 }
 
+int bdrv_enable_write_cache(BlockDriverState *bs)
+{
+    return bs->enable_write_cache;
+}
+
 /* XXX: no longer used */
 void bdrv_set_change_cb(BlockDriverState *bs,
                         void (*change_cb)(void *opaque), void *opaque)
diff --git a/block.h b/block.h
index ea6905291e..0ae2526531 100644
--- a/block.h
+++ b/block.h
@@ -135,6 +135,7 @@ int bdrv_get_translation_hint(BlockDriverState *bs);
 int bdrv_is_removable(BlockDriverState *bs);
 int bdrv_is_read_only(BlockDriverState *bs);
 int bdrv_is_sg(BlockDriverState *bs);
+int bdrv_enable_write_cache(BlockDriverState *bs);
 int bdrv_is_inserted(BlockDriverState *bs);
 int bdrv_media_changed(BlockDriverState *bs);
 int bdrv_is_locked(BlockDriverState *bs);
diff --git a/block_int.h b/block_int.h
index 3e4b4a387b..f95c80b906 100644
--- a/block_int.h
+++ b/block_int.h
@@ -158,6 +158,9 @@ struct BlockDriverState {
     /* the memory alignment required for the buffers handled by this driver */
     int buffer_alignment;
 
+    /* do we need to tell the quest if we have a volatile write cache? */
+    int enable_write_cache;
+
     /* NOTE: the following infos are only hints for real hardware
        drivers. They are not used by the block driver */
     int cyls, heads, secs, translation;
diff --git a/hw/ide/core.c b/hw/ide/core.c
index fe5bd172af..6fd6dc2477 100644
--- a/hw/ide/core.c
+++ b/hw/ide/core.c
@@ -148,8 +148,11 @@ static void ide_identify(IDEState *s)
     put_le16(p + 83, (1 << 14) | (1 << 13) | (1 <<12) | (1 << 10));
     /* 14=set to 1, 1=SMART self test, 0=SMART error logging */
     put_le16(p + 84, (1 << 14) | 0);
-    /* 14 = NOP supported, 0=SMART feature set enabled */
-    put_le16(p + 85, (1 << 14) | 1);
+    /* 14 = NOP supported, 5=WCACHE enabled, 0=SMART feature set enabled */
+    if (bdrv_enable_write_cache(s->bs))
+         put_le16(p + 85, (1 << 14) | (1 << 5) | 1);
+    else
+         put_le16(p + 85, (1 << 14) | 1);
     /* 13=flush_cache_ext,12=flush_cache,10=lba48 */
     put_le16(p + 86, (1 << 14) | (1 << 13) | (1 <<12) | (1 << 10));
     /* 14=set to 1, 1=smart self test, 0=smart error logging */
diff --git a/hw/scsi-disk.c b/hw/scsi-disk.c
index ffc2654bf9..de8e314b15 100644
--- a/hw/scsi-disk.c
+++ b/hw/scsi-disk.c
@@ -709,7 +709,9 @@ static int32_t scsi_send_command(SCSIDevice *d, uint32_t tag,
                 memset(p,0,20);
                 p[0] = 8;
                 p[1] = 0x12;
-                p[2] = 4; /* WCE */
+                if (bdrv_enable_write_cache(s->dinfo->bdrv)) {
+                     p[2] = 4; /* WCE */
+                }
                 p += 20;
             }
             if ((page == 0x3f || page == 0x2a)