Re: [PATCH v8 1/5] QIOChannel: Add flags on io_writev and introduce io_flush callback

[Daniel P . Berrangé]

On Tue, Feb 01, 2022 at 03:28:59AM -0300, Leonardo Bras wrote:
> Add flags to io_writev and introduce io_flush as optional callback to
> QIOChannelClass, allowing the implementation of zero copy writes by
> subclasses.
> 
> How to use them:
> - Write data using qio_channel_writev*(...,QIO_CHANNEL_WRITE_FLAG_ZERO_COPY),
> - Wait write completion with qio_channel_flush().
> 
> Notes:
> As some zero copy write implementations work asynchronously, it's
> recommended to keep the write buffer untouched until the return of
> qio_channel_flush(), to avoid the risk of sending an updated buffer
> instead of the buffer state during write.
> 
> As io_flush callback is optional, if a subclass does not implement it, then:
> - io_flush will return 0 without changing anything.
> 
> Also, some functions like qio_channel_writev_full_all() were adapted to
> receive a flag parameter. That allows shared code between zero copy and
> non-zero copy writev, and also an easier implementation on new flags.
> 
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  include/io/channel.h                | 38 ++++++++++++++++++++-
>  chardev/char-io.c                   |  2 +-
>  hw/remote/mpqemu-link.c             |  2 +-
>  io/channel-buffer.c                 |  1 +
>  io/channel-command.c                |  1 +
>  io/channel-file.c                   |  1 +
>  io/channel-socket.c                 |  2 ++
>  io/channel-tls.c                    |  1 +
>  io/channel-websock.c                |  1 +
>  io/channel.c                        | 53 +++++++++++++++++++++++------
>  migration/rdma.c                    |  1 +
>  scsi/pr-manager-helper.c            |  2 +-
>  tests/unit/test-io-channel-socket.c |  1 +
>  13 files changed, 92 insertions(+), 14 deletions(-)
> 
> diff --git a/io/channel.c b/io/channel.c
> index e8b019dc36..b8b99fdc4c 100644
> --- a/io/channel.c
> +++ b/io/channel.c
> @@ -72,18 +72,32 @@ ssize_t qio_channel_writev_full(QIOChannel *ioc,
>                                  size_t niov,
>                                  int *fds,
>                                  size_t nfds,
> +                                int flags,
>                                  Error **errp)
>  {
>      QIOChannelClass *klass = QIO_CHANNEL_GET_CLASS(ioc);
>  
> -    if ((fds || nfds) &&
> -        !qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_FD_PASS)) {
> +    if (fds || nfds) {
> +        if (!qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_FD_PASS)) {
> +            error_setg_errno(errp, EINVAL,
> +                             "Channel does not support file descriptor 
> passing");
> +            return -1;
> +        }
> +        if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> +            error_setg_errno(errp, EINVAL,
> +                             "Zero Copy does not support file descriptor 
> passing");
> +            return -1;
> +        }

Here you gracefully reject FD passing when zero copy is requested
which is good.

> +    }
> +

> @@ -235,10 +249,16 @@ int qio_channel_writev_full_all(QIOChannel *ioc,
>                            iov, niov,
>                            0, iov_size(iov, niov));
>  
> +    if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> +        assert(fds == NULL && nfds == 0);
> +    }

But here you  abort QEMU if FD passing is requested when zero copy
is set.

AFAICT, if you just delete this assert, the code to gracefully
report errors will do the right thing.

Without the assert:

  Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

Regards,
Daniel
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