Re: [Qemu-devel] [PATCH 1/6] block: Keep track of devices' I/O status

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 23 Sep 2011 09:51:24 +0200
Markus Armbruster <armbru@redhat.com> wrote:

> Luiz Capitulino <lcapitulino@redhat.com> writes:
> 
> > This commit adds support to the BlockDriverState type to keep track
> > of devices' I/O status.
> >
> > There are three possible status: BDRV_IOS_OK (no error), BDRV_IOS_ENOSPC
> > (no space error) and BDRV_IOS_FAILED (any other error). The distinction
> > between no space and other errors is important because a management
> > application may want to watch for no space in order to extend the
> > space assigned to the VM and put it to run again.
> >
> > Qemu devices supporting the I/O status feature have to enable it
> > explicitly by calling bdrv_iostatus_enable() _and_ have to be
> > configured to stop the VM on errors (ie. werror=stop|enospc or
> > rerror=stop).
> >
> > In case of multiple errors being triggered in sequence only the first
> > one is stored. The I/O status is always reset to BDRV_IOS_OK when the
> > 'cont' command is issued.
> >
> > Next commits will add support to some devices and extend the
> > query-block/info block commands to return the I/O status information.
> >
> > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > ---
> >  block.c     |   32 ++++++++++++++++++++++++++++++++
> >  block.h     |    9 +++++++++
> >  block_int.h |    1 +
> >  monitor.c   |    6 ++++++
> >  4 files changed, 48 insertions(+), 0 deletions(-)
> >
> > diff --git a/block.c b/block.c
> > index e3fe97f..fbd90b4 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -221,6 +221,7 @@ BlockDriverState *bdrv_new(const char *device_name)
> >      if (device_name[0] != '\0') {
> >          QTAILQ_INSERT_TAIL(&bdrv_states, bs, list);
> >      }
> > +    bs->iostatus = BDRV_IOS_INVAL;
> >      return bs;
> >  }
> >  
> > @@ -3181,6 +3182,37 @@ int bdrv_in_use(BlockDriverState *bs)
> >      return bs->in_use;
> >  }
> >  
> > +void bdrv_iostatus_enable(BlockDriverState *bs)
> > +{
> > +    assert(bs->iostatus == BDRV_IOS_INVAL);
> > +    bs->iostatus = BDRV_IOS_OK;
> > +}
> 
> bdrv_new() creates the BDS with I/O status tracking disabled.  Devices
> that do tracking declare that by calling this function during
> initialization.  Enables tracking if the BDS has suitable on_write_error
> and on_read_error.
> 
> If a device gets hot unplugged, tracking remains enabled.  If the BDS
> then gets reused with a device that doesn't do tracking, I/O status
> becomes incorrect.  Can't happen right now, because we automatically
> delete the BDS on hot unplug, but it's a trap.
> 
> Suggest to disable tracking in bdrv_detach_dev() or bdrv_attach_dev().

Ok, and in bdrv_close() as Zhi Yong said.

> > +
> > +/* The I/O status is only enabled if the drive explicitly
> > + * enables it _and_ the VM is configured to stop on errors */
> > +bool bdrv_iostatus_is_enabled(const BlockDriverState *bs)
> > +{
> > +    return (bs->iostatus != BDRV_IOS_INVAL &&
> > +           (bs->on_write_error == BLOCK_ERR_STOP_ENOSPC ||
> > +            bs->on_write_error == BLOCK_ERR_STOP_ANY    ||
> > +            bs->on_read_error == BLOCK_ERR_STOP_ANY));
> > +}
> > +
> > +void bdrv_iostatus_reset(BlockDriverState *bs)
> > +{
> > +    if (bdrv_iostatus_is_enabled(bs)) {
> > +        bs->iostatus = BDRV_IOS_OK;
> > +    }
> > +}
> > +
> > +void bdrv_iostatus_set_err(BlockDriverState *bs, int error)
> > +{
> > +    if (bdrv_iostatus_is_enabled(bs) && bs->iostatus == BDRV_IOS_OK) {
> > +        bs->iostatus = (abs(error) == ENOSPC) ? BDRV_IOS_ENOSPC :
> > +                                                BDRV_IOS_FAILED;
> > +    }
> > +}
> > +
> 
> abs(error) feels... unusual.
> 
> If you want to guard against callers passing wrongly signed values, why
> not simply assert(error >= 0)?

Yes. Actually, I thought that there were existing devices that could pass
a positive value (while others passed a negative one), but by taking a look
at the code now it seems that all supported devices pass a negative value,
so your suggestion makes sense.

> 
> >  void
> >  bdrv_acct_start(BlockDriverState *bs, BlockAcctCookie *cookie, int64_t bytes,
> >          enum BlockAcctType type)
> > diff --git a/block.h b/block.h
> > index 16bfa0a..de74af0 100644
> > --- a/block.h
> > +++ b/block.h
> > @@ -77,6 +77,15 @@ typedef enum {
> >      BDRV_ACTION_REPORT, BDRV_ACTION_IGNORE, BDRV_ACTION_STOP
> >  } BlockMonEventAction;
> >  
> > +typedef enum {
> > +    BDRV_IOS_INVAL, BDRV_IOS_OK, BDRV_IOS_FAILED, BDRV_IOS_ENOSPC,
> > +    BDRV_IOS_MAX
> > +} BlockIOStatus;
> 
> Why is this in block.h?

I followed BlockErrorAction, you suggest block_int.h?

> 
> > +
> > +void bdrv_iostatus_enable(BlockDriverState *bs);
> > +void bdrv_iostatus_reset(BlockDriverState *bs);
> > +bool bdrv_iostatus_is_enabled(const BlockDriverState *bs);
> > +void bdrv_iostatus_set_err(BlockDriverState *bs, int error);
> >  void bdrv_mon_event(const BlockDriverState *bdrv,
> >                      BlockMonEventAction action, int is_read);
> >  void bdrv_info_print(Monitor *mon, const QObject *data);
> > diff --git a/block_int.h b/block_int.h
> > index 8c3b863..f2f4f2d 100644
> > --- a/block_int.h
> > +++ b/block_int.h
> > @@ -199,6 +199,7 @@ struct BlockDriverState {
> >         drivers. They are not used by the block driver */
> >      int cyls, heads, secs, translation;
> >      BlockErrorAction on_read_error, on_write_error;
> > +    BlockIOStatus iostatus;
> >      char device_name[32];
> >      unsigned long *dirty_bitmap;
> >      int64_t dirty_count;
> > diff --git a/monitor.c b/monitor.c
> > index 8ec2c5e..88d8228 100644
> > --- a/monitor.c
> > +++ b/monitor.c
> > @@ -1304,6 +1304,11 @@ struct bdrv_iterate_context {
> >      int err;
> >  };
> >  
> > +static void iostatus_bdrv_it(void *opaque, BlockDriverState *bs)
> > +{
> > +    bdrv_iostatus_reset(bs);
> > +}
> > +
> >  /**
> >   * do_cont(): Resume emulation.
> >   */
> > @@ -1320,6 +1325,7 @@ static int do_cont(Monitor *mon, const QDict *qdict, QObject **ret_data)
> >          return -1;
> >      }
> >  
> > +    bdrv_iterate(iostatus_bdrv_it, NULL);
> >      bdrv_iterate(encrypted_bdrv_it, &context);
> >      /* only resume the vm if all keys are set and valid */
> >      if (!context.err) {
>