Libguestfs - Feb 2022 - [PATCH nbdkit] blocksize: Export block size constraints

This filter is a little unusual because it allows clients to send a
wider range of request sizes than the underlying plugin allows.
Therefore we advertise the widest possible minimum and maximum block
size to clients.

We still need to pick a suitable preferred block size assuming the
plugin itself doesn't advertise one.
---
 filters/blocksize/blocksize.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/filters/blocksize/blocksize.c b/filters/blocksize/blocksize.c
index a6fa00cb..46e759b0 100644
--- a/filters/blocksize/blocksize.c
+++ b/filters/blocksize/blocksize.c
@@ -156,6 +156,29 @@ blocksize_get_size (nbdkit_next *next,
   return ROUND_DOWN (size, minblock);
 }
 
+/* Block size constraints.
+ *
+ * Note that the purpose of this filter is to allow clients to send a
+ * wider range of request sizes than the underlying plugin permits,
+ * and therefore this callback advertises the full availability of the
+ * filter, likely widening the constraints from the plugin.
+ */
+static int
+blocksize_block_size (nbdkit_next *next, void *handle,
+                      uint32_t *minimum, uint32_t *preferred, uint32_t
*maximum)
+{
+  if (next->block_size (next, minimum, preferred, maximum) == -1)
+    return -1;
+
+  if (*preferred == 0)
+    *preferred = MAX (4096, minblock);
+
+  *minimum = 1;
+  *maximum = 0xffffffff;
+
+  return 0;
+}
+
 static int
 blocksize_pread (nbdkit_next *next,
                  void *handle, void *b, uint32_t count, uint64_t offs,
@@ -432,6 +455,7 @@ static struct nbdkit_filter filter = {
   .config_complete   = blocksize_config_complete,
   .config_help       = blocksize_config_help,
   .get_size          = blocksize_get_size,
+  .block_size        = blocksize_block_size,
   .pread             = blocksize_pread,
   .pwrite            = blocksize_pwrite,
   .trim              = blocksize_trim,
-- 
2.35.1

On 02/20/22 21:56, Richard W.M. Jones wrote:
> This filter is a little unusual because it allows clients to send a
> wider range of request sizes than the underlying plugin allows.
> Therefore we advertise the widest possible minimum and maximum block
> size to clients.
> 
> We still need to pick a suitable preferred block size assuming the
> plugin itself doesn't advertise one.
> ---
>  filters/blocksize/blocksize.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/filters/blocksize/blocksize.c b/filters/blocksize/blocksize.c
> index a6fa00cb..46e759b0 100644
> --- a/filters/blocksize/blocksize.c
> +++ b/filters/blocksize/blocksize.c
> @@ -156,6 +156,29 @@ blocksize_get_size (nbdkit_next *next,
>    return ROUND_DOWN (size, minblock);
>  }
>  
> +/* Block size constraints.
> + *
> + * Note that the purpose of this filter is to allow clients to send a
> + * wider range of request sizes than the underlying plugin permits,
> + * and therefore this callback advertises the full availability of the
> + * filter, likely widening the constraints from the plugin.
> + */
> +static int
> +blocksize_block_size (nbdkit_next *next, void *handle,
> +                      uint32_t *minimum, uint32_t *preferred, uint32_t
*maximum)
> +{
> +  if (next->block_size (next, minimum, preferred, maximum) == -1)
> +    return -1;
> +
> +  if (*preferred == 0)
> +    *preferred = MAX (4096, minblock);
> +
> +  *minimum = 1;
> +  *maximum = 0xffffffff;
> +
> +  return 0;
> +}
> +
>  static int
>  blocksize_pread (nbdkit_next *next,
>                   void *handle, void *b, uint32_t count, uint64_t offs,
> @@ -432,6 +455,7 @@ static struct nbdkit_filter filter = {
>    .config_complete   = blocksize_config_complete,
>    .config_help       = blocksize_config_help,
>    .get_size          = blocksize_get_size,
> +  .block_size        = blocksize_block_size,
>    .pread             = blocksize_pread,
>    .pwrite            = blocksize_pwrite,
>    .trim              = blocksize_trim,
> 
After reading the manual...

Acked-by: Laszlo Ersek <lersek at redhat.com>

On Sun, Feb 20, 2022 at 08:56:37PM +0000, Richard W.M. Jones
wrote:
> This filter is a little unusual because it allows clients to send a
> wider range of request sizes than the underlying plugin allows.
> Therefore we advertise the widest possible minimum and maximum block
> size to clients.
> 
> We still need to pick a suitable preferred block size assuming the
> plugin itself doesn't advertise one.
> ---
>  filters/blocksize/blocksize.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/filters/blocksize/blocksize.c b/filters/blocksize/blocksize.c
> index a6fa00cb..46e759b0 100644
> --- a/filters/blocksize/blocksize.c
> +++ b/filters/blocksize/blocksize.c
> @@ -156,6 +156,29 @@ blocksize_get_size (nbdkit_next *next,
>    return ROUND_DOWN (size, minblock);
>  }
>  
> +/* Block size constraints.
> + *
> + * Note that the purpose of this filter is to allow clients to send a
> + * wider range of request sizes than the underlying plugin permits,
> + * and therefore this callback advertises the full availability of the
> + * filter, likely widening the constraints from the plugin.
> + */
> +static int
> +blocksize_block_size (nbdkit_next *next, void *handle,
> +                      uint32_t *minimum, uint32_t *preferred, uint32_t
*maximum)
> +{
> +  if (next->block_size (next, minimum, preferred, maximum) == -1)
> +    return -1;
> +
> +  if (*preferred == 0)
> +    *preferred = MAX (4096, minblock);
Hmm.  Even if minblock > *maximum reported by the plugin,...
> +
> +  *minimum = 1;
> +  *maximum = 0xffffffff;
...our advertisement to the client does not trigger any asserts.

ACK.

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