blockdev: enable discard by default

Because discard is now a host parameter, we can always fake it as enabled
in the guest.  This is an extension of the current choice to ignore
"not supported" errors from the host when discard_granularity is set
to nonzero.

The default granularity is set to the logical block size or 4k, whichever
is largest, because cluster sizes below 4k are rarely used and 4K is a
typical block size for files.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

hw/block-common.h

@@ -50,7 +50,7 @@ static inline unsigned int get_physical_block_exp(BlockConf *conf)
     DEFINE_PROP_UINT32("opt_io_size", _state, _conf.opt_io_size, 0),    \
     DEFINE_PROP_INT32("bootindex", _state, _conf.bootindex, -1),        \
     DEFINE_PROP_UINT32("discard_granularity", _state, \
-                       _conf.discard_granularity, 0)
+                       _conf.discard_granularity, -1)
 
 #define DEFINE_BLOCK_CHS_PROPERTIES(_state, _conf)      \
     DEFINE_PROP_UINT32("cyls", _state, _conf.cyls, 0),  \

hw/ide/qdev.c

@@ -143,7 +143,10 @@ static int ide_dev_initfn(IDEDevice *dev, IDEDriveKind kind)
     IDEBus *bus = DO_UPCAST(IDEBus, qbus, dev->qdev.parent_bus);
     IDEState *s = bus->ifs + dev->unit;
 
-    if (dev->conf.discard_granularity && dev->conf.discard_granularity != 512) {
+    if (dev->conf.discard_granularity == -1) {
+        dev->conf.discard_granularity = 512;
+    } else if (dev->conf.discard_granularity &&
+               dev->conf.discard_granularity != 512) {
         error_report("discard_granularity must be 512 for ide");
         return -1;
     }

hw/scsi-disk.c

@@ -41,9 +41,11 @@ do { printf("scsi-disk: " fmt , ## __VA_ARGS__); } while (0)
 #include <scsi/sg.h>
 #endif
 
-#define SCSI_DMA_BUF_SIZE    131072
-#define SCSI_MAX_INQUIRY_LEN 256
-#define SCSI_MAX_MODE_LEN    256
+#define SCSI_DMA_BUF_SIZE           131072
+#define SCSI_MAX_INQUIRY_LEN        256
+#define SCSI_MAX_MODE_LEN           256
+
+#define DEFAULT_DISCARD_GRANULARITY 4096
 
 typedef struct SCSIDiskState SCSIDiskState;
 
@@ -2059,6 +2061,11 @@ static int scsi_initfn(SCSIDevice *dev)
         return -1;
     }
 
+    if (s->qdev.conf.discard_granularity == -1) {
+        s->qdev.conf.discard_granularity =
+            MAX(s->qdev.conf.logical_block_size, DEFAULT_DISCARD_GRANULARITY);
+    }
+
     if (!s->version) {
         s->version = g_strdup(qemu_get_version());
     }