linux-kernel - Re: [RFC PATCH v5 14/17] docs: net: description of MSG_ZEROCOPY for AF

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Message-ID: <CAGxU2F410NSNSzdNS4m-9UM8rZFBFpe5LeNZtkF0VzJc5_JFmg@mail.gmail.com>
Date:   Thu, 6 Jul 2023 19:06:17 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <AVKrasnov@...rdevices.ru>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Bobby Eshleman <bobby.eshleman@...edance.com>,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...rdevices.ru, oxffffaa@...il.com
Subject: Re: [RFC PATCH v5 14/17] docs: net: description of MSG_ZEROCOPY for AF_VSOCK

On Sat, Jul 01, 2023 at 09:39:44AM +0300, Arseniy Krasnov wrote:
>This adds description of MSG_ZEROCOPY flag support for AF_VSOCK type of
>socket.
>
>Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
>---
> Documentation/networking/msg_zerocopy.rst | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
>diff --git a/Documentation/networking/msg_zerocopy.rst b/Documentation/networking/msg_zerocopy.rst
>index b3ea96af9b49..34bc7ff411ce 100644
>--- a/Documentation/networking/msg_zerocopy.rst
>+++ b/Documentation/networking/msg_zerocopy.rst
>@@ -7,7 +7,8 @@ Intro
> =====
>
> The MSG_ZEROCOPY flag enables copy avoidance for socket send calls.
>-The feature is currently implemented for TCP and UDP sockets.
>+The feature is currently implemented for TCP, UDP and VSOCK (with
>+virtio transport) sockets.
>
>
> Opportunity and Caveats
>@@ -174,7 +175,7 @@ read_notification() call in the previous snippet. A notification
> is encoded in the standard error format, sock_extended_err.
>
> The level and type fields in the control data are protocol family
>-specific, IP_RECVERR or IPV6_RECVERR.
>+specific, IP_RECVERR or IPV6_RECVERR (for TCP or UDP socket).
>
> Error origin is the new type SO_EE_ORIGIN_ZEROCOPY. ee_errno is zero,
> as explained before, to avoid blocking read and write system calls on
>@@ -201,6 +202,7 @@ undefined, bar for ee_code, as discussed below.
>
>       printf("completed: %u..%u\n", serr->ee_info, serr->ee_data);
>
>+For VSOCK socket, cmsg_level will be SOL_VSOCK and cmsg_type will be 0.

Maybe better to move up, just under the previous change.

By the way, should we define a valid type value for vsock
(e.g. VSOCK_RECVERR)?

>
> Deferred copies
> ~~~~~~~~~~~~~~~
>@@ -235,12 +237,15 @@ Implementation
> Loopback
> --------
>
>+For TCP and UDP:
> Data sent to local sockets can be queued indefinitely if the receive
> process does not read its socket. Unbound notification latency is not
> acceptable. For this reason all packets generated with MSG_ZEROCOPY
> that are looped to a local socket will incur a deferred copy. This
> includes looping onto packet sockets (e.g., tcpdump) and tun devices.
>
>+For VSOCK:
>+Data path sent to local sockets is the same as for non-local sockets.
>
> Testing
> =======
>@@ -254,3 +259,6 @@ instance when run with msg_zerocopy.sh between a veth pair across
> namespaces, the test will not show any improvement. For testing, the
> loopback restriction can be temporarily relaxed by making
> skb_orphan_frags_rx identical to skb_orphan_frags.
>+
>+For VSOCK type of socket example can be found in  tools/testing/vsock/
>+vsock_test_zerocopy.c.

For VSOCK socket, example can be found in
tools/testing/vsock/vsock_test_zerocopy.c

(we should leave the entire path on the same line)

>--
>2.25.1
>