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Wed, 01 Sep 2021 08:52:16 -0700 (PDT) X-Google-Smtp-Source: ABdhPJwOISP5goCAI+5PYD3QNyXBJ+3K8sowzBxXSTEMze/qlAghQDapzKE5qY60ROz24/TqgcFP7A== X-Received: by 2002:a37:b606:: with SMTP id g6mr300006qkf.476.1630511535922; Wed, 01 Sep 2021 08:52:15 -0700 (PDT) Received: from t490s ([2607:fea8:56a3:500::ad7f]) by smtp.gmail.com with ESMTPSA id x2sm115111qtr.6.2021.09.01.08.52.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 01 Sep 2021 08:52:15 -0700 (PDT) Date: Wed, 1 Sep 2021 11:52:13 -0400 From: Peter Xu To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Subject: Re: [PATCH v1 2/3] io: Add zerocopy and errqueue Message-ID: References: <20210831110238.299458-1-leobras@redhat.com> <20210831110238.299458-3-leobras@redhat.com> MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.392, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Elena Ufimtseva , John G Johnson , Jagannathan Raman , qemu-block@nongnu.org, Juan Quintela , "Dr. David Alan Gilbert" , qemu-devel@nongnu.org, Leonardo Bras , Paolo Bonzini , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , Fam Zheng Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Sep 01, 2021 at 09:50:56AM +0100, Daniel P. Berrangé wrote: > On Tue, Aug 31, 2021 at 04:27:04PM -0400, Peter Xu wrote: > > On Tue, Aug 31, 2021 at 01:57:33PM +0100, Daniel P. Berrangé wrote: > > > On Tue, Aug 31, 2021 at 08:02:38AM -0300, Leonardo Bras wrote: > > > > MSG_ZEROCOPY is a feature that enables copy avoidance in TCP/UDP socket > > > > send calls. It does so by avoiding copying user data into kernel buffers. > > > > > > > > To make it work, three steps are needed: > > > > 1 - A setsockopt() system call, enabling SO_ZEROCOPY > > > > 2 - Passing down the MSG_ZEROCOPY flag for each send*() syscall > > > > 3 - Process the socket's error queue, dealing with any error > > > > > > AFAICT, this is missing the single most critical aspect of MSG_ZEROCOPY. > > > > > > It is non-obvious, but setting the MSG_ZEROCOPY flag turns sendmsg() > > > from a synchronous call to an asynchronous call. > > > > > > It is forbidden to overwrite/reuse/free the buffer passed to sendmsg > > > until an asynchronous completion notification has been received from > > > the socket error queue. These notifications are not required to > > > arrive in-order, even for a TCP stream, because the kernel hangs on > > > to the buffer if a re-transmit is needed. > > > > > > https://www.kernel.org/doc/html/v5.4/networking/msg_zerocopy.html > > > > > > "Page pinning also changes system call semantics. It temporarily > > > shares the buffer between process and network stack. Unlike with > > > copying, the process cannot immediately overwrite the buffer > > > after system call return without possibly modifying the data in > > > flight. Kernel integrity is not affected, but a buggy program > > > can possibly corrupt its own data stream." > > > > > > AFAICT, the design added in this patch does not provide any way > > > to honour these requirements around buffer lifetime. > > > > > > I can't see how we can introduce MSG_ZEROCOPY in any seemless > > > way. The buffer lifetime requirements imply need for an API > > > design that is fundamentally different for asynchronous usage, > > > with a callback to notify when the write has finished/failed. > > > > Regarding buffer reuse - it indeed has a very deep implication on the buffer > > being available and it's not obvious at all. Just to mention that the initial > > user of this work will make sure all zero copy buffers will be guest pages only > > (as it's only used in multi-fd), so they should always be there during the > > process. > > That is not the case when migration is using TLS, because the buffers > transmitted are the ciphertext buffer, not the plaintext guest page. Agreed. > > > In short, we may just want to at least having a way to make sure all zero > > copied buffers are finished using and they're sent after some function returns > > (e.g., qio_channel_flush()). That may require us to do some accounting on when > > we called sendmsg(MSG_ZEROCOPY), meanwhile we should need to read out the > > ee_data field within SO_EE_ORIGIN_ZEROCOPY msg when we do recvmsg() for the > > error queue and keep those information somewhere too. > > > > Some other side notes that reached my mind.. > > > > The qio_channel_writev_full() may not be suitable for async operations, as the > > name "full" implies synchronous to me. So maybe we can add a new helper for > > zero copy on the channel? > > All the APIs that exist today are fundamentally only suitable for sync > operations. Supporting async correctly will definitely a brand new APIs > separate from what exists today. Yes. What I wanted to say is maybe we can still keep the io_writev() interface untouched, but just add a new interface at qio_channel_writev_full() level. IOW, we could comment on io_writev() that it can be either sync or async now, just like sendmsg() has that implication too with different flag passed in. When calling io_writev() with different upper helpers, QIO channel could identify what flag to pass over to io_writev(). -- Peter Xu