Re: [PATCH v4 3/3] multifd: Implement zerocopy write in multifd migration (multifd-zerocopy)

[Peter Xu]

On Sat, Oct 09, 2021 at 04:56:13AM -0300, Leonardo Bras wrote:
> @@ -105,7 +105,13 @@ static int nocomp_send_prepare(MultiFDSendParams *p, 
> uint32_t used,
>   */
>  static int nocomp_send_write(MultiFDSendParams *p, uint32_t used, Error 
> **errp)
>  {
> -    return qio_channel_writev_all(p->c, p->pages->iov, used, errp);
> +    int flags = 0;
> +
> +    if (migrate_multifd_zerocopy()) {
> +        flags = QIO_CHANNEL_WRITE_FLAG_ZEROCOPY;
> +    }
> +
> +    return qio_channel_writev_all_flags(p->c, p->pages->iov, used, flags, 
> errp);
>  }

What if the user specified ZEROCOPY+MULTIFD, but the kernel doesn't support it?

IIUC then the first call to qio_channel_writev_all_flags() above will fail,
then we fail the migration.

It seems fine, but since you've introduced QIO_CHANNEL_FEATURE_WRITE_ZEROCOPY
in the previous patch - I think the cleaner way is when start migration and
after we setup the ioc, we sanity check on the capability and the ioc to make
sure if ZEROCOPY+MULTIFD is specified, we should fail early if we're sure the
ioc doesn't have QIO_CHANNEL_FEATURE_WRITE_ZEROCOPY bit set?

-- 
Peter Xu