The block backend changed in a way that flushing empty CDROM drives
is now an error. Amend IDE to avoid doing so until the root problem
can be addressed for 2.11.

Reported-by: Kieron Shorrock <kshorrock@paloaltonetworks.com>
Signed-off-by: John Snow <jsnow@redhat.com>
---
 hw/ide/core.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/hw/ide/core.c b/hw/ide/core.c
index 0b48b64..6cbca43 100644
--- a/hw/ide/core.c
+++ b/hw/ide/core.c
@@ -1053,17 +1053,21 @@ static void ide_flush_cb(void *opaque, int ret)
     ide_set_irq(s->bus);
 }
 
-static void ide_flush_cache(IDEState *s)
+static bool ide_flush_cache(IDEState *s)
 {
     if (s->blk == NULL) {
         ide_flush_cb(s, 0);
-        return;
+        return false;
+    } else if (!blk_bs(s->blk)) {
+        /* Nothing to flush */
+        return true;
     }
 
     s->status |= BUSY_STAT;
     ide_set_retry(s);
     block_acct_start(blk_get_stats(s->blk), &s->acct, 0, BLOCK_ACCT_FLUSH);
     s->pio_aiocb = blk_aio_flush(s->blk, ide_flush_cb, s);
+    return false;
 }
 
 static void ide_cfata_metadata_inquiry(IDEState *s)
@@ -1508,8 +1512,7 @@ static bool cmd_write_dma(IDEState *s, uint8_t cmd)
 
 static bool cmd_flush_cache(IDEState *s, uint8_t cmd)
 {
-    ide_flush_cache(s);
-    return false;
+    return ide_flush_cache(s);
 }
 
 static bool cmd_seek(IDEState *s, uint8_t cmd)
-- 
2.9.4

On 08/08/2017 12:57 PM, John Snow wrote:
> The block backend changed in a way that flushing empty CDROM drives
> is now an error. Amend IDE to avoid doing so until the root problem
> can be addressed for 2.11.
> 
> Reported-by: Kieron Shorrock <kshorrock@paloaltonetworks.com>
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  hw/ide/core.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

On Tue, Aug 08, 2017 at 01:57:08PM -0400, John Snow wrote:
> The block backend changed in a way that flushing empty CDROM drives
> is now an error. Amend IDE to avoid doing so until the root problem
> can be addressed for 2.11.
> 
> Reported-by: Kieron Shorrock <kshorrock@paloaltonetworks.com>
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  hw/ide/core.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/ide/core.c b/hw/ide/core.c
> index 0b48b64..6cbca43 100644
> --- a/hw/ide/core.c
> +++ b/hw/ide/core.c
> @@ -1053,17 +1053,21 @@ static void ide_flush_cb(void *opaque, int ret)
>      ide_set_irq(s->bus);
>  }
>  
> -static void ide_flush_cache(IDEState *s)
> +static bool ide_flush_cache(IDEState *s)

Previously this function invoked ide_flush_cb() to complete the request
and raise an IRQ.  Now it may return true instead of invoking
ide_flush_cb().  It's no longer a helper function, it's more like an IDE
command handler.

cmd_set_features() must be updated:

  case 0x82: /* write cache disable */
      blk_set_enable_write_cache(s->blk, false);
      identify_data = (uint16_t *)s->identify_data;
      put_le16(identify_data + 85, (1 << 14) | 1);
      ide_flush_cache(s);
      return false;  <--- should be "return ide_flush_cache(s)"

To make the code clearer I suggest deleting ide_flush_cache() and
calling cmd_flush_cache() directly instead.  That way it's obvious that
this is an IDE command handler and it may return true to indicate the
the command completed immediately.

>  {
>      if (s->blk == NULL) {
>          ide_flush_cb(s, 0);
> -        return;
> +        return false;
> +    } else if (!blk_bs(s->blk)) {
> +        /* Nothing to flush */

Please move information from the commit description into this comment if
you respin.  Someone looking at the code might think this is a nop that
is safe to remove.  Once blk_*() is fixed this code path can be removed
again.