Re: [PATCH v3 04/14] io: Add generic pwritev/preadv interface

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Oct 28, 2022 at 01:39:04PM +0300, Nikolay Borisov wrote:
> Introduce basic pwriteve/preadv support in the generic channel layer.
> SPecific implementation will follow for the file channel as this is
> required in order to support migration streams with fixed location of
> each ram page.
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  include/io/channel.h | 49 ++++++++++++++++++++++++++++++++++++++++++++
>  io/channel.c         | 26 +++++++++++++++++++++++
>  2 files changed, 75 insertions(+)
> 
> diff --git a/include/io/channel.h b/include/io/channel.h
> index c680ee748021..6b10bce8bbdf 100644
> --- a/include/io/channel.h
> +++ b/include/io/channel.h
> @@ -124,6 +124,16 @@ struct QIOChannelClass {
>                             Error **errp);
>  
>      /* Optional callbacks */
> +    ssize_t (*io_pwritev)(QIOChannel *ioc,
> +                       const struct iovec *iov,
> +                       size_t niov,
> +                       off_t offset,
> +                       Error **errp);
> +    ssize_t (*io_preadv)(QIOChannel *ioc,
> +                      const struct iovec *iov,
> +                      size_t niov,
> +                      off_t offset,
> +                      Error **errp);

nit-pick the alignment of the 2nd line of parameters onwards,
needs to be indented by 3 more spaces.


> +/**
> + * qio_channel_io_pwritev
> + * @ioc: the channel object
> + * @iov: the array of memory regions to write data from
> + * @niov: the length of the @iov array
> + * @offset: offset in the channel where writes should begin
> + * @errp: pointer to a NULL-initialized error object
> + *
> + * Not all implementations will support this facility, so may report an error.
> + * To avoid errors, the caller may check for the feature flag
> + * QIO_CHANNEL_FEATURE_SEEKABLE prior to calling this method.
> + *
> + * Behaves as qio_channel_writev_full, apart from not supporting sending of file

Given this, we should have  "_full" suffix on these methods
too for consistent naming policy.

> + * handles as well as beginning the write at the passed @offset
> + *
> + */
> +ssize_t qio_channel_io_pwritev(QIOChannel *ioc, const struct iovec *iov,
> +                               size_t niov, off_t offset, Error **errp);
> +
> +
> +/**
> + * qio_channel_io_preadv
> + * @ioc: the channel object
> + * @iov: the array of memory regions to read data into
> + * @niov: the length of the @iov array
> + * @offset: offset in the channel where writes should begin
> + * @errp: pointer to a NULL-initialized error object
> + *
> + * Not all implementations will support this facility, so may report an error.
> + * To avoid errors, the caller may check for the feature flag
> + * QIO_CHANNEL_FEATURE_SEEKABLE prior to calling this method.
> + *
> + * Behaves as qio_channel_readv_full, apart from not supporting receiving of file
> + * handles as well as beginning the read at the passed @offset
> + *
> + */
> +ssize_t qio_channel_io_preadv(QIOChannel *ioc, const struct iovec *iov,
> +                             size_t niov, off_t offset, Error **errp);
>  /**
>   * qio_channel_shutdown:
>   * @ioc: the channel object
> diff --git a/io/channel.c b/io/channel.c
> index 0640941ac573..f5ac9499a7ad 100644
> --- a/io/channel.c
> +++ b/io/channel.c
> @@ -437,6 +437,32 @@ GSource *qio_channel_add_watch_source(QIOChannel *ioc,
>  }
>  
>  
> +ssize_t qio_channel_io_pwritev(QIOChannel *ioc, const struct iovec *iov,
> +                               size_t niov, off_t offset, Error **errp)
> +{
> +    QIOChannelClass *klass = QIO_CHANNEL_GET_CLASS(ioc);
> +
> +    if (!klass->io_pwritev) {
> +        error_setg(errp, "Channel does not support pwritev");
> +        return -1;
> +    }

This also possibly benefits from a validation check

    if (!qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_SEEKABLE)) {
        error_setg_errno(errp, EINVAL,
                         "Requested channel is not seekable")
        return -1;
    }

because the QIOChannelFile impl will support io_pwritev callback,
but not all the FDs it can use will support seeking.

Same check for preadv

> +
> +    return klass->io_pwritev(ioc, iov, niov, offset, errp);
> +}
> +
> +ssize_t qio_channel_io_preadv(QIOChannel *ioc, const struct iovec *iov,
> +                               size_t niov, off_t offset, Error **errp)
> +{
> +    QIOChannelClass *klass = QIO_CHANNEL_GET_CLASS(ioc);
> +
> +    if (!klass->io_preadv) {
> +        error_setg(errp, "Channel does not support preadv");
> +        return -1;
> +    }
> +
> +    return klass->io_preadv(ioc, iov, niov, offset, errp);
> +}
> +
>  int qio_channel_shutdown(QIOChannel *ioc,
>                           QIOChannelShutdown how,
>                           Error **errp)
> -- 
> 2.34.1
> 

With regards,
Daniel
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