Re: [PATCH v8 3/7] block: add block layer APIs resembling Linux ZonedBlockDevice ioctls

[Sam Li]

Markus Armbruster <armbru@redhat.com> 于2022年8月31日周三 16:35写道：
>
> Sam Li <faithilikerun@gmail.com> writes:
>
> > Markus Armbruster <armbru@redhat.com> 于2022年8月30日周二 19:57写道：
> >>
> >> Sam Li <faithilikerun@gmail.com> writes:
> >>
> >> > By adding zone management operations in BlockDriver, storage controller
> >> > emulation can use the new block layer APIs including Report Zone and
> >> > four zone management operations (open, close, finish, reset).
> >> >
> >> > Add zoned storage commands of the device: zone_report(zrp), 
> >> > zone_open(zo),
> >> > zone_close(zc), zone_reset(zrs), zone_finish(zf).
> >> >
> >> > For example, to test zone_report, use following command:
> >> > $ ./build/qemu-io --image-opts driver=zoned_host_device, 
> >> > filename=/dev/nullb0
> >> > -c "zrp offset nr_zones"
> >> >
> >> > Signed-off-by: Sam Li <faithilikerun@gmail.com>
> >> > Reviewed-by: Hannes Reinecke <hare@suse.de>
> >>
> >> [...]
> >>
> >> > diff --git a/block/file-posix.c b/block/file-posix.c
> >> > index 0a8b4b426e..e3efba6db7 100644
> >> > --- a/block/file-posix.c
> >> > +++ b/block/file-posix.c
> >>
> >> [...]
> >>
> >> > @@ -3752,6 +4025,54 @@ static BlockDriver bdrv_host_device = {
> >> >  #endif
> >> >  };
> >> >
> >> > +#if defined(CONFIG_BLKZONED)
> >> > +static BlockDriver bdrv_zoned_host_device = {
> >> > +        .format_name = "zoned_host_device",
> >>
> >> Indentation should be 4, not 8.
> >>
> >> > +        .protocol_name = "zoned_host_device",
> >> > +        .instance_size = sizeof(BDRVRawState),
> >> > +        .bdrv_needs_filename = true,
> >> > +        .bdrv_probe_device  = hdev_probe_device,
> >> > +        .bdrv_file_open     = hdev_open,
> >> > +        .bdrv_close         = raw_close,
> >> > +        .bdrv_reopen_prepare = raw_reopen_prepare,
> >> > +        .bdrv_reopen_commit  = raw_reopen_commit,
> >> > +        .bdrv_reopen_abort   = raw_reopen_abort,
> >> > +        .bdrv_co_create_opts = bdrv_co_create_opts_simple,
> >> > +        .create_opts         = &bdrv_create_opts_simple,
> >> > +        .mutable_opts        = mutable_opts,
> >> > +        .bdrv_co_invalidate_cache = raw_co_invalidate_cache,
> >> > +        .bdrv_co_pwrite_zeroes = hdev_co_pwrite_zeroes,
> >> > +
> >> > +        .bdrv_co_preadv         = raw_co_preadv,
> >> > +        .bdrv_co_pwritev        = raw_co_pwritev,
> >> > +        .bdrv_co_flush_to_disk  = raw_co_flush_to_disk,
> >> > +        .bdrv_co_pdiscard       = hdev_co_pdiscard,
> >> > +        .bdrv_co_copy_range_from = raw_co_copy_range_from,
> >> > +        .bdrv_co_copy_range_to  = raw_co_copy_range_to,
> >> > +        .bdrv_refresh_limits = raw_refresh_limits,
> >> > +        .bdrv_io_plug = raw_aio_plug,
> >> > +        .bdrv_io_unplug = raw_aio_unplug,
> >> > +        .bdrv_attach_aio_context = raw_aio_attach_aio_context,
> >> > +
> >> > +        .bdrv_co_truncate       = raw_co_truncate,
> >> > +        .bdrv_getlength = raw_getlength,
> >> > +        .bdrv_get_info = raw_get_info,
> >> > +        .bdrv_get_allocated_file_size
> >> > +                            = raw_get_allocated_file_size,
> >> > +        .bdrv_get_specific_stats = hdev_get_specific_stats,
> >> > +        .bdrv_check_perm = raw_check_perm,
> >> > +        .bdrv_set_perm   = raw_set_perm,
> >> > +        .bdrv_abort_perm_update = raw_abort_perm_update,
> >> > +        .bdrv_probe_blocksizes = hdev_probe_blocksizes,
> >> > +        .bdrv_probe_geometry = hdev_probe_geometry,
> >> > +        .bdrv_co_ioctl = hdev_co_ioctl,
> >> > +
> >> > +        /* zone management operations */
> >> > +        .bdrv_co_zone_report = raw_co_zone_report,
> >> > +        .bdrv_co_zone_mgmt = raw_co_zone_mgmt,
> >> > +};
> >>
> >> Differences to bdrv_host_device:
> >>
> >> * .bdrv_parse_filename is not set
> >>
> >> * .bdrv_co_ioctl is not set
> >>
> >> * .bdrv_co_zone_report and .bdrv_co_zone_mgmt are set
> >
> > As Stefan mentioned, zoned_host_device is a new driver that doesn't
> > work with string filenames. .bdrv_parse_filename() helps legacy
> > drivers strip the optional protocol prefix off the filename and no use
> > here. Therefore it can be dropped.
>
> Makes sense.
>
> > .bdrv_co_ioctl is set actually.
>
> You're right; I diffed the two and misread the result.
>
> > Zoned_host_device is basically host_device + zone operations. It
> > serves for a simple purpose: if the host device is zoned, register
> > zoned_host_device driver; else, register host_device.
>
> Why would I ever want to use host_device instead of zoned_host_device?
>
> To answer this question, we need to understand how their behavior
> differs.
>
> We can ignore the legacy protocol prefix / string filename part.
>
> All that's left seems to be "if the host device is zoned, then using the
> zoned_host_device driver gets you the zoned features, whereas using the
> host_device driver doesn't".  What am I missing?

I think that's basically what users need to know about.

>
> >> Notably common is .bdrv_file_open = hdev_open.  What happens when you
> >> try to create a zoned_host_device where the @filename argument is not in
> >> fact a zoned device?
> >
> > If the device is a regular block device, QEMU will still open the
> > device. For instance, I use a loopback device to test zone_report in
> > qemu-io. It returns ENOTTY which indicates Inappropriate ioctl for the
> > device. Meanwhile, if using a regular block device when emulation a
> > zoned device on a guest os, the best case is that the guest can boot
> > but has no emulated block device. In some cases, QEMU just terminates
> > because the block device has not met the alignment requirements.
>
> I'm not sure I understand all of this.  I'm also not sure I have to :)

Maybe I didn't explain it very well. Which part would you like to know
more about?

>
> >> Do we really need a separate, but almost identical BlockDriver?  Could
> >> the existing one provide zoned functionality exactly when the underlying
> >> host device does?
> >
> > I did use the existing one host device to add zoned commands at first.
> > But then, we decided to change that and use a separate BlockDriver.
> > Though the existing one can provide zoned functionality, a new
> > BlockDriver makes it clear when mixing block drivers, adding more
> > configurations/constraints, etc. For example, zoned devices must
> > enforce direct I/O instead of using page cache to ensure the order of
> > writes. It would be good to print a message for users when using
> > zoned_host_device without setting direct I/O.
> >
> > However, it's still a simple version I was thinking about and can be
> > improved/changed afterward. Using host_device only is possible I think
> > but needs more carefully thinking.
>
> I'm not opposed to making this a separate driver.  But the case for it
> should be made in the commit message.  Discussing it in review is a fine
> way to get to a better commit message, of course.

That's great! I'll mention the zoned_host device BlockDriver in the
commit message of the next revision.


Thanks for reviewing. If I missed anything, please tell me.

>
> > Maybe Damien and Stefan can talk more about this?
> >
> >>
> >> Forgive me if these are ignorant questions, or have been discussed
> >> before.
> >
> > Always a pleasure.
>
> Thanks!
>
> [...]
>