Re: [Qemu-devel] [Libguestfs] [nbdkit PATCH 3/3] plugins: Add .can_fast_zero hook

[Eric Blake <eblake@redhat.com>]

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On 8/23/19 9:40 AM, Eric Blake wrote:
> Allow plugins to affect the handling of the new NBD_CMD_FLAG_FAST_ZERO
> flag, then update affected plugins.  In particular, in-memory plugins
> are always fast; the full plugin is better served by omitting .zero
> and relying on .pwrite fallback; nbd takes advantage of libnbd
> extensions proposed in parallel to pass through support; and
> v2 language bindings expose the choice to their scripts.
> 

> +++ b/server/plugins.c
> @@ -404,11 +404,25 @@ plugin_can_zero (struct backend *b, struct connection *conn)
>  static int
>  plugin_can_fast_zero (struct backend *b, struct connection *conn)
>  {
> +  struct backend_plugin *p = container_of (b, struct backend_plugin, backend);
> +  int r;
> +
>    assert (connection_get_handle (conn, 0));
> 
>    debug ("can_fast_zero");
> 
> -  return 0; /* Upcoming patch will actually add support. */
> +  if (p->plugin.can_fast_zero)
> +    return p->plugin.can_fast_zero (connection_get_handle (conn, 0));
> +  /* Advertise support for fast zeroes if no .zero or .can_zero is
> +   * false: in those cases, we fail fast instead of using .pwrite.
> +   * This also works when v1 plugin has only ._zero_old.
> +   */
> +  if (p->plugin.zero == NULL)
> +    return 1;
> +  r = plugin_can_zero (b, conn);
> +  if (r == -1)
> +    return -1;
> +  return !r;
>  }
> 

Needs this squashed in for libnbd to pass rather than skip its new
'can-fast-zero-flag' test:


diff --git i/server/plugins.c w/server/plugins.c
index 84329cf4..695a77ab 100644
--- i/server/plugins.c
+++ w/server/plugins.c
@@ -208,6 +208,7 @@ plugin_dump_fields (struct backend *b)
   HAS (can_cache);
   HAS (cache);
   HAS (thread_model);
+  HAS (can_fast_zero);
 #undef HAS

   /* Custom fields. */


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org


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