Re: [Xen-devel] [PATCH] xen-block: only advertize discard to the frontend when it is enabled...

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Anthony PERARD [mailto:anthony.perard@xxxxxxxxxx]
> Sent: 21 March 2019 11:42
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; qemu-block@xxxxxxxxxx; 
> qemu-devel@xxxxxxxxxx; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Kevin Wolf <kwolf@xxxxxxxxxx>; Max Reitz 
> <mreitz@xxxxxxxxxx>
> Subject: Re: [PATCH] xen-block: only advertize discard to the frontend when 
> it is enabled...
> 
> On Wed, Mar 20, 2019 at 02:28:25PM +0000, Paul Durrant wrote:
> > ...and properly enable it when synthesizing a drive.
> >
> > The Xen toolstack sets 'discard-enable' to '1' in xenstore when it wants
> > to enable discard on a specified image. The code in
> > xen_block_driver_create() correctly parses this and uses it to set
> 
> typo: It's xen_block_drive_create (s/driver/drive/), otherwise my IDE
> can't find it :-(.

Sorry, my fingers are so used to typing 'driver'

> 
> > 'discard' to 'unamp' for the file_layer, but fails to do the same for the
> 
> Looks like s/unamp/unmap/

Yes.

> 
> > driver_layer (which effectively disables it). Meanwhile the code in
> > xen_block_realize() advertizes discard support to the frontend in the
> > default case (because conf->discard_granularity defaults to -1), even when
> 
> That doesn't match the code I'm reading, before the patch apply.
>     if (discard_granularity > 0) feature-discard=1
> Nothing seems to set discard_granularity, so it keeps it's default to
> -1, so .... wait, discard_granularity is unsigned :-(
> 
> The description is fine then.

Indeed.

> 
> > the underlying image may not handle it.
> >
> > This patch adds the missing option to the driver_layer in
> > xen_block_driver_create() and checks whether BDRV_O_UNMAP is actually
> > set on the block device before advertizing discard to the frontend.
> > In the case that discard is supported it also makes sure that the
> > granularity is set to the physical block size.
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> 
> With the two typos fixed (which I can try to remember to do on commit):
> Reviewed-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>

Ok, thanks.

  Paul

> 
> Thanks,
> 
> --
> Anthony PERARD

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