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Re: [PATCH v4 2/3] QIOChannelSocket: Implement io_writev_zerocopy & io_f
From: |
Eric Blake |
Subject: |
Re: [PATCH v4 2/3] QIOChannelSocket: Implement io_writev_zerocopy & io_flush_zerocopy for CONFIG_LINUX |
Date: |
Mon, 11 Oct 2021 14:27:10 -0500 |
User-agent: |
NeoMutt/20210205-852-339c0c |
On Sat, Oct 09, 2021 at 04:56:12AM -0300, Leonardo Bras wrote:
> For CONFIG_LINUX, implement the new optional callbacks io_write_zerocopy and
> io_flush_zerocopy on QIOChannelSocket, but enables it only when MSG_ZEROCOPY
> feature is available in the host kernel, which is checked on
> qio_channel_socket_connect_sync()
>
> qio_channel_socket_writev() contents were moved to a helper function
> qio_channel_socket_writev_flags() which accepts an extra argument for flags.
> (This argument is passed directly to sendmsg().
>
> The above helper function is used to implement qio_channel_socket_writev(),
> with flags = 0, keeping it's behavior unchanged, and
its (remember, "it's" is shorthand for "it is", which does not fit here)
> qio_channel_socket_writev_zerocopy() with flags = MSG_ZEROCOPY.
>
> qio_channel_socket_flush_zerocopy() was implemented by counting how many times
> sendmsg(...,MSG_ZEROCOPY) was sucessfully called, and then reading the
> socket's error queue, in order to find how many of them finished sending.
> Flush will loop until those counters are the same, or until some error ocurs.
occurs
>
> A new function qio_channel_socket_poll() was also created in order to avoid
> busy-looping recvmsg() in qio_channel_socket_flush_zerocopy() while waiting
> for
> updates in socket's error queue.
>
> Notes on using writev_zerocopy():
> 1: Buffer
> - As MSG_ZEROCOPY tells the kernel to use the same user buffer to avoid
> copying,
> some caution is necessary to avoid overwriting any buffer before it's sent.
> If something like this happen, a newer version of the buffer may be sent
> instead.
> - If this is a problem, it's recommended to call flush_zerocopy() before
> freeing
> or re-using the buffer.
>
> 2: Locked memory
> - When using MSG_ZERCOCOPY, the buffer memory will be locked after queued, and
> unlocked after it's sent.
> - Depending on the size of each buffer, and how often it's sent, it may
> require
> a larger amount of locked memory than usually available to non-root user.
> - If the required amount of locked memory is not available, writev_zerocopy
> will return an error, which can abort an operation like migration,
> - Because of this, when an user code wants to add zerocopy as a feature, it
> requires a mechanism to disable it, so it can still be acessible to less
> privileged users.
>
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
> include/io/channel-socket.h | 2 +
> include/io/channel.h | 1 +
> io/channel-socket.c | 180 ++++++++++++++++++++++++++++++++++--
> 3 files changed, 173 insertions(+), 10 deletions(-)
>
> +static int qio_channel_socket_flush_zerocopy(QIOChannel *ioc,
> + Error **errp)
> +{
> +
> + /* No errors, count sucessfully finished sendmsg()*/
Space before */
> + sioc->zerocopy_sent += serr->ee_data - serr->ee_info + 1;
> + }
> + return 0;
> +}
> +
> +#endif /* CONFIG_LINUX */
> +
> static int
> qio_channel_socket_set_blocking(QIOChannel *ioc,
> bool enabled,
> @@ -787,6 +943,10 @@ static void qio_channel_socket_class_init(ObjectClass
> *klass,
> ioc_klass->io_set_delay = qio_channel_socket_set_delay;
> ioc_klass->io_create_watch = qio_channel_socket_create_watch;
> ioc_klass->io_set_aio_fd_handler = qio_channel_socket_set_aio_fd_handler;
> +#ifdef CONFIG_LINUX
> + ioc_klass->io_writev_zerocopy = qio_channel_socket_writev_zerocopy;
> + ioc_klass->io_flush_zerocopy = qio_channel_socket_flush_zerocopy;
> +#endif
> }
I did a high-level look at the code, rather than an in-depth review of
whether zero-copy was being used correctly.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
- Re: [PATCH v4 1/3] QIOChannel: Add io_writev_zerocopy & io_flush_zerocopy callbacks, (continued)
Re: [PATCH v4 1/3] QIOChannel: Add io_writev_zerocopy & io_flush_zerocopy callbacks, Peter Xu, 2021/10/13
[PATCH v4 2/3] QIOChannelSocket: Implement io_writev_zerocopy & io_flush_zerocopy for CONFIG_LINUX, Leonardo Bras, 2021/10/09
[PATCH v4 3/3] multifd: Implement zerocopy write in multifd migration (multifd-zerocopy), Leonardo Bras, 2021/10/09
- Re: [PATCH v4 3/3] multifd: Implement zerocopy write in multifd migration (multifd-zerocopy), Eric Blake, 2021/10/11
- Re: [PATCH v4 3/3] multifd: Implement zerocopy write in multifd migration (multifd-zerocopy), Leonardo Bras Soares Passos, 2021/10/11
- Re: [PATCH v4 3/3] multifd: Implement zerocopy write in multifd migration (multifd-zerocopy), Markus Armbruster, 2021/10/12
- Re: [PATCH v4 3/3] multifd: Implement zerocopy write in multifd migration (multifd-zerocopy), Leonardo Bras Soares Passos, 2021/10/27
- Re: [PATCH v4 3/3] multifd: Implement zerocopy write in multifd migration (multifd-zerocopy), Markus Armbruster, 2021/10/28
- Re: [PATCH v4 3/3] multifd: Implement zerocopy write in multifd migration (multifd-zerocopy), Leonardo Bras Soares Passos, 2021/10/28