[Libguestfs] [nbdkit PATCH] swab: Implement .block_size callback
Laszlo Ersek
lersek at redhat.com
Tue Feb 22 11:49:13 UTC 2022
On 02/21/22 23:00, Eric Blake wrote:
> We were previously enforcing minimum block size with EINVAL for
> too-small requests. Advertise this to the client.
> ---
> filters/swab/nbdkit-swab-filter.pod | 6 ++++++
> filters/swab/swab.c | 24 +++++++++++++++++++++++-
> 2 files changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/filters/swab/nbdkit-swab-filter.pod b/filters/swab/nbdkit-swab-filter.pod
> index f8500150..030a0852 100644
> --- a/filters/swab/nbdkit-swab-filter.pod
> +++ b/filters/swab/nbdkit-swab-filter.pod
> @@ -34,6 +34,11 @@ the last few bytes, combine this filter with
> L<nbdkit-truncate-filter(1)>; fortunately, sector-based disk images
> are already suitably sized.
>
> +Note that this filter fails operations that are not aligned to the
> +swab-bits boundaries; if you need byte-level access, apply the
> +L<nbdkit-blocksize-filter(1)> before this one, to get
> +read-modify-write access to individual bytes.
> +
> =head1 PARAMETERS
I understand that the alignment of requests is enforced, but what
happens if the client sends a request (correctly aligned) that is 17
bytes in size, for example?
... Aha, so is_aligned() doesn't only check "offset", it also checks
"count". That wasn't clear to me from the addition to
"filters/swab/nbdkit-swab-filter.pod". I suggest spelling that out more
explicitly.
>
> =over 4
> @@ -90,6 +95,7 @@ L<nbdkit(1)>,
> L<nbdkit-file-plugin(1)>,
> L<nbdkit-pattern-plugin(1)>,
> L<nbdkit-filter(3)>,
> +L<nbdkit-blocksize-filter(1)>.
> L<nbdkit-truncate-filter(1)>.
>
> =head1 AUTHORS
> diff --git a/filters/swab/swab.c b/filters/swab/swab.c
> index 68776eee..6e8dc981 100644
> --- a/filters/swab/swab.c
> +++ b/filters/swab/swab.c
> @@ -44,6 +44,7 @@
> #include "byte-swapping.h"
> #include "isaligned.h"
> #include "cleanup.h"
> +#include "minmax.h"
> #include "rounding.h"
>
> /* Can only be 8 (filter disabled), 16, 32 or 64. */
> @@ -85,8 +86,28 @@ swab_get_size (nbdkit_next *next,
> return ROUND_DOWN (size, bits/8);
> }
>
> +/* Block size constraints. */
> +static int
> +swab_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 (*minimum == 0) { /* No constraints set by the plugin. */
> + *minimum = bits/8;
> + *preferred = 512;
> + *maximum = 0xffffffff;
> + }
> + else {
> + *minimum = MAX (*minimum, bits/8);
> + }
Given that the count too must be a whole multiple of the swap-block size
(correctly so), what if the underlying plugin specifies a minimum block
size of 17?
I think that will take effect here, and then this filter will specify
such a minimum block size (17) that it will, in turn, reject
unconditionally. That kind of defeats the purpose of exposing a "minimum
block size".
Wouldn't it be better if, on the "else" branch, we rounded up "*minimum"?
*minimum = ROUND_UP (*minimum, bits/8);
Thanks,
Laszlo
> +
> + return 0;
> +}
> +
> /* The request must be aligned.
> - * XXX We could lift this restriction with more work.
> + * If you want finer alignment, use the blocksize filter.
> */
> static bool
> is_aligned (uint32_t count, uint64_t offset, int *err)
> @@ -220,6 +241,7 @@ static struct nbdkit_filter filter = {
> .config = swab_config,
> .config_help = swab_config_help,
> .get_size = swab_get_size,
> + .block_size = swab_block_size,
> .pread = swab_pread,
> .pwrite = swab_pwrite,
> .trim = swab_trim,
>
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