FITRIM is a mounted filesystem feature to discard (or "trim") blocks which
are not in use by the filesystem. This is useful for solid-state drives
(SSDs) and thinly-provisioned storage.  Provide access to the feature
from the host so that filesystems can be trimmed periodically or before
migration.

Here is an example using scsi_debug:

    # modprobe scsi_debug lbpu=1 lbpws=1
    # sg_vpd -p0xb2 /dev/sdb
    Logical block provisioning VPD page (SBC):
      Unmap command supported (LBPU): 1
      Write same (16) with unmap bit supported (LBWS): 1
      Write same (10) with unmap bit supported (LBWS10): 0
    # mke2fs /dev/sdb
    # cat /sys/bus/pseudo/drivers/scsi_debug/map
    1-616,16257-16383
    # mount /dev/sdb /run/media/pbonzini/test
    # dd if=/dev/zero of=/run/media/pbonzini/test/file
    # cat map
    1-616,645-1588,1599-4026,4029-16383
    # rm /run/media/pbonzini/test/file
    # ./qemu-ga /dev/fd/0
    {"execute":"guest-fstrim"}
    {"return": {}}
    # cat map
    1-612

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 qapi-schema-guest.json |   20 ++++++++++++++
 qga/commands-posix.c   |   72 +++++++++++++++++++++++++++++++++++++++++++++++-
 qga/commands-win32.c   |   11 ++++++++
 3 files changed, 102 insertions(+), 1 deletion(-)

diff --git a/qapi-schema-guest.json b/qapi-schema-guest.json
index d4055d2..72bedac 100644
--- a/qapi-schema-guest.json
+++ b/qapi-schema-guest.json
@@ -351,6 +351,26 @@
   'returns': 'int' }
 
 ##
+# @guest-fstrim:
+#
+# Discard (or "trim") blocks which are not in use by the filesystem.
+#
+# @minimum:
+#       Minimum contiguous free range to discard, in bytes. Free ranges
+#       smaller than this may be ignored (this is a hint and the guest
+#       may not respect it).  By increasing this value, the fstrim
+#       operation will complete more quickly for filesystems with badly
+#       fragmented free space, although not all blocks will be discarded.
+#       The default value is zero, meaning "discard every free block".
+#
+# Returns: Nothing.
+#
+# Since: 0.15.0
+##
+{ 'command': 'guest-fstrim',
+  'data': { '*minimum': 'int' } }
+
+##
 # @guest-suspend-disk
 #
 # Suspend guest to disk.
diff --git a/qga/commands-posix.c b/qga/commands-posix.c
index b1a7ce6..802d13a 100644
--- a/qga/commands-posix.c
+++ b/qga/commands-posix.c
@@ -38,9 +38,12 @@ extern char **environ;
 #include <sys/socket.h>
 #include <net/if.h>
 
-#if defined(__linux__) && defined(FIFREEZE)
+#ifdef FIFREEZE
 #define CONFIG_FSFREEZE
 #endif
+#ifdef FITRIM
+#define CONFIG_FSTRIM
+#endif
 #endif
 
 void qmp_guest_shutdown(bool has_mode, const char *mode, Error **err)
@@ -525,6 +528,65 @@ static void guest_fsfreeze_cleanup(void)
 }
 #endif /* CONFIG_FSFREEZE */
 
+#if defined(CONFIG_FSTRIM)
+/*
+ * Walk list of mounted file systems in the guest, and trim them.
+ */
+void qmp_guest_fstrim(bool has_minimum, int64_t minimum, Error **err)
+{
+    int ret = 0;
+    FsMountList mounts;
+    struct FsMount *mount;
+    int fd;
+    char err_msg[512];
+    struct fstrim_range r = {
+        .start = 0,
+        .len = -1,
+        .minlen = has_minimum ? minimum : 0,
+    };
+
+    slog("guest-fstrim called");
+
+    QTAILQ_INIT(&mounts);
+    ret = build_fs_mount_list(&mounts);
+    if (ret < 0) {
+        return;
+    }
+
+    QTAILQ_FOREACH(mount, &mounts, next) {
+        fd = qemu_open(mount->dirname, O_RDONLY);
+        if (fd == -1) {
+            sprintf(err_msg, "failed to open %s, %s", mount->dirname,
+                    strerror(errno));
+            error_set(err, QERR_QGA_COMMAND_FAILED, err_msg);
+            goto error;
+        }
+
+        /* We try to cull filesytems we know won't work in advance, but other
+         * filesytems may not implement fstrim for less obvious reasons.  these
+         * will report EOPNOTSUPP. we simply ignore these errors.  Any other
+         * error means an unexpected error, so return it in those cases.  In
+         * some other cases ENOTTY will be reported (e.g. CD-ROMs).
+         */
+        ret = ioctl(fd, FITRIM, &r);
+        if (ret == -1) {
+            if (errno != ENOTTY && errno != EOPNOTSUPP) {
+                sprintf(err_msg, "failed to trim %s, %s",
+                        mount->dirname, strerror(errno));
+                error_set(err, QERR_QGA_COMMAND_FAILED, err_msg);
+                close(fd);
+                goto error;
+            }
+        }
+        close(fd);
+    }
+
+error:
+    free_fs_mount_list(&mounts);
+}
+#endif /* CONFIG_FSTRIM */
+
+
 #define LINUX_SYS_STATE_FILE "/sys/power/state"
 #define SUSPEND_SUPPORTED 0
 #define SUSPEND_NOT_SUPPORTED 1
@@ -918,7 +980,15 @@ int64_t qmp_guest_fsfreeze_thaw(Error **err)
 
     return 0;
 }
+#endif /* CONFIG_FSFREEZE */
 
+#if !defined(CONFIG_FSTRIM)
+void qmp_guest_fstrim(bool has_minimum, int64_t minimum, Error **err)
+{
+    error_set(err, QERR_UNSUPPORTED);
+
+    return;
+}
 #endif
 
 /* register init/cleanup routines for stateful command groups */
diff --git a/qga/commands-win32.c b/qga/commands-win32.c
index eb8d140..54bc546 100644
--- a/qga/commands-win32.c
+++ b/qga/commands-win32.c
@@ -173,6 +173,17 @@ int64_t qmp_guest_fsfreeze_thaw(Error **err)
     return 0;
 }
 
+/*
+ * Walk list of mounted file systems in the guest, and discard unused
+ * areas.
+ */
+void qmp_guest_fstrim(bool has_minimum, int64_t minimum, Error **err)
+{
+    error_set(err, QERR_UNSUPPORTED);
+
+    return;
+}
+
 typedef enum {
     GUEST_SUSPEND_MODE_DISK,
     GUEST_SUSPEND_MODE_RAM
-- 
1.7.10.2