Re: [PATCH 1/2] block: Expose block limits for images in QMP

[Kevin Wolf <kwolf@redhat.com>]

Am 24.09.2025 um 08:10 hat Markus Armbruster geschrieben:
> Kevin Wolf <kwolf@redhat.com> writes:
> 
> > This information can be useful both for debugging and for management
> > tools trying to configure guest devices with the optimal limits
> > (possibly across multiple hosts). There is no reason not to make it
> > available, so just add it to BlockNodeInfo.
> >
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >  qapi/block-core.json             | 59 ++++++++++++++++++++++++++++++++
> >  block/qapi.c                     | 34 ++++++++++++++++--
> >  tests/qemu-iotests/184           |  3 +-
> >  tests/qemu-iotests/184.out       |  8 -----
> >  tests/qemu-iotests/common.filter |  3 +-
> >  5 files changed, 94 insertions(+), 13 deletions(-)
> >
> > diff --git a/qapi/block-core.json b/qapi/block-core.json
> > index dc6eb4ae23..eda041ac1c 100644
> > --- a/qapi/block-core.json
> > +++ b/qapi/block-core.json
> > @@ -275,6 +275,62 @@
> >        'file': 'ImageInfoSpecificFileWrapper'
> >    } }
> >  
> > +##
> > +# @BlockLimitsInfo:
> > +#
> > +# @request-alignment: Alignment requirement, in bytes, for offset/length of I/O
> > +#     requests.
> > +#
> > +# @max-discard: Maximum number of bytes that can be discarded at once. If not
> > +#     present, there is no specific maximum.
> > +#
> > +# @discard-alignment: Optimal alignment for discard requests in bytes. A power
> > +#     of 2 is best, but not mandatory. If not present, discards don't have a
> > +#     alignment requirement different from @request-alignment.
> 
> What does the second sentence try to convey?  As far as I can tell, QMP
> has BlockLimitsInfo is only in the result of query-block and
> query-named-block-nodes, i.e. it's not something the user picks.

I copied these descriptions from the comments in struct BlockLimits,
just leaving out things that are clearly internal. Their nature is the
same there, we never configure block limits, we only detect them.

What I think this sentence wants to tell us is that while you may
intuitively expect power-of-two limits, you shouldn't be surprised to
occasionally find other numbers here, too.

Maybe "Note that this doesn't have to be a power of two" instead? Both
in QAPI and the struct definition.

> > +#
> > +# @max-write-zeroes: Maximum number of bytes that can be zeroed out at once. If
> > +#     not present, there is no specific maximum.
> > +#
> > +# @write-zeroes-alignment: Optimal alignment for write_zeroes requests in
> > +#     bytes. A power of 2 is best, but not mandatory. If not present,
> > +#     write_zeroes doesn't have a alignment requirement different from
> > +#     @request-alignment.
> 
> Likewise.
> 
> > +#
> > +# @opt-transfer: Optimal transfer length in bytes. If not present, there is no
> > +#     preferred size.
> > +#
> > +# @max-transfer: Maximal transfer length in bytes. If not present, there is no
> > +#     specific maximum.
> > +#
> > +# @max-hw-transfer: Maximal hardware transfer length in bytes.  Applies
> > +#     whenever transfers to the device bypass the kernel I/O scheduler, for
> > +#     example with SG_IO. If not present, there is no specific maximum.
> > +#
> > +# @max-iov: Maximum number of scatter/gather elements
> > +#
> > +# @max-hw-iov: Maximal number of scatter/gather elements allowed by the hardware.
> 
> Maximum number
> 
> > +#     Applies whenever transfers to the device bypass the kernel I/O scheduler,
> > +#     for example with SG_IO. If not present, the hardware limits is unknown
> > +#     and @max-iov is always used.
> > +#
> > +# @min-mem-alignment: memory alignment in bytes so that no bounce buffer is needed
> > +#
> > +# @opt-mem-alignment: memory alignment in bytes that is used for bounce buffers
> 
> Why is this "opt"?  I guess it means "optimal".

Yes, I think so. How about this:

@min-mem-alignment: Minimal required memory alignment in bytes for
zero-copy I/O to succeed. For unaligned requrests, a bounce buffer will
be used.

@opt-mem-alignment: Optimal memory alignment in bytes. This is the
alignment used for any buffer allocations QEMU performs internally.

> > +##
> > +{ 'struct': 'BlockLimitsInfo',
> > +  'data': { 'request-alignment': 'uint32',
> > +            '*max-discard': 'uint64',
> > +            '*discard-alignment': 'uint32',
> > +            '*max-write-zeroes': 'uint64',
> > +            '*write-zeroes-alignment': 'uint32',
> > +            '*opt-transfer': 'uint32',
> > +            '*max-transfer': 'uint32',
> > +            '*max-hw-transfer': 'uint32',
> > +            'max-iov': 'int',
> > +            '*max-hw-iov': 'int',
> > +            'min-mem-alignment': 'size',
> > +            'opt-mem-alignment': 'size' } }
> > +
> >  ##
> >  # @BlockNodeInfo:
> >  #
> > @@ -304,6 +360,8 @@
> >  #
> >  # @snapshots: list of VM snapshots
> >  #
> > +# @limits: block limits that are used for I/O on the node (Since 10.2)
> > +#
> >  # @format-specific: structure supplying additional format-specific
> >  #     information (since 1.7)
> >  #
> > @@ -315,6 +373,7 @@
> >             '*cluster-size': 'int', '*encrypted': 'bool', '*compressed': 'bool',
> >             '*backing-filename': 'str', '*full-backing-filename': 'str',
> >             '*backing-filename-format': 'str', '*snapshots': ['SnapshotInfo'],
> > +           '*limits': 'BlockLimitsInfo',
> >             '*format-specific': 'ImageInfoSpecific' } }
> >  
> >  ##
> 
> docs/devel/qapi-code-gen.rst:
> 
>     For legibility, wrap text paragraphs so every line is at most 70
>     characters long.
> 
>     Separate sentences with two spaces.

It would be better to use a uniform text width across the project that
can be trivially configured in every editor than wasting our brain
cycles on manually enforcing an idiosyncratic style in certain parts of
certain files.

I'll make the change for this patch, but without tool support, I'm
almost sure that this will happen again and again.

Kevin