Re: [Qemu-devel] [Qemu-block] [PATCH] block: Simplify bdrv_can_write_zeroes_with_unmap()

[Eric Blake]

On 01/29/2018 05:08 AM, Stefan Hajnoczi wrote:
> On Fri, Jan 26, 2018 at 01:34:39PM -0600, Eric Blake wrote:
> > We don't need the can_write_zeroes_with_unmap field in
> > BlockDriverInfo, because it is redundant information with
> > supported_zero_flags & BDRV_REQ_MAY_UNMAP.  Note that
> > BlockDriverInfo and supported_zero_flags are both per-device
> > settings, rather than global state about the driver as a
> > whole, which means one or both of these bits of information
> > can already be conditional.  Let's audit how they were set:
...
> > Simplify the code by moving the conditional into
> > supported_zero_flags for all drivers, then dropping the
> > now-unused BDI field.  For callers that relied on
> > bdrv_can_write_zeroes_with_unmap(), we return the same
> > per-device settings for drivers that had conditions (no
> > observable change in behavior there); and can now return
> > true (instead of false) for drivers that support passthrough
> > (for example, the commit driver) which gives those drivers
> > the same fix as nbd just got in bca80059e.  For callers that
> > relied on supported_zero_flags, we now have a few more places
> > that can avoid a wasted call to pwrite_zeroes() that will
> > just fail with ENOTSUP.
> >
> > Suggested-by: Paolo Bonzini <address@hidden>
> > Signed-off-by: Eric Blake <address@hidden>
> >
> > ---
> > The commit id mentioned above is dependent on me not having to respin
> > my latest NBD pull request:
> >
> > Based-on: <address@hidden>
> > ([PULL 0/8] NBD patches through 26 Jan)

> Reviewed-by: Stefan Hajnoczi <address@hidden>

Thanks.

Since this patch was discovered in relation to NBD code, I'm fine taking 
it through my NBD queue; but it's also more generic to the block layer, 
so I'm also fine if one of the other block maintainers grabs it first 
through their tree.

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