From: Leonardo Bras Soares Passos <leobras@redhat.com>
To: "Daniel P. Berrangé" <berrange@redhat.com>
Cc: "Elena Ufimtseva" <elena.ufimtseva@oracle.com>,
"John G Johnson" <john.g.johnson@oracle.com>,
"Jagannathan Raman" <jag.raman@oracle.com>,
qemu-block@nongnu.org, "Juan Quintela" <quintela@redhat.com>,
"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
"Peter Xu" <peterx@redhat.com>,
qemu-devel <qemu-devel@nongnu.org>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Marc-André Lureau" <marcandre.lureau@redhat.com>,
"Fam Zheng" <fam@euphon.net>
Subject: Re: [PATCH v1 2/3] io: Add zerocopy and errqueue
Date: Thu, 2 Sep 2021 04:07:39 -0300 [thread overview]
Message-ID: <CAJ6HWG6tSSYwesTyitPmiSDTMuiqkk4YeBzptni+46GPU9zKow@mail.gmail.com> (raw)
In-Reply-To: <YS8+8EKboNvsB0zP@redhat.com>
Hello Daniel,.
A few more comments:
On Wed, Sep 1, 2021 at 5:51 AM Daniel P. Berrangé <berrange@redhat.com> 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.
That makes sense.
I am still working my way on Migration code, and I ended up not
knowing that part.
I think we can work on KTLS for this case, and implement something
'like zerocopy' for this. I added more details in the previous mail I sent you.
>
> > 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.
That's a great, I was thinking on implementing this as an optional method for
QIOChannel, more details in the previous mail :).
(sorry, I ended up reading/replying the mails in thread order)
>
> Regards,
> Daniel
> --
> |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org -o- https://fstop138.berrange.com :|
> |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
>
Best regards,
Leonardo Bras
next prev parent reply other threads:[~2021-09-02 7:20 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-31 11:02 [PATCH v1 0/3] QIOChannel flags + multifd zerocopy Leonardo Bras
2021-08-31 11:02 ` [PATCH v1 1/3] io: Enable write flags for QIOChannel Leonardo Bras
2021-09-01 20:54 ` Eric Blake
2021-09-02 8:26 ` Leonardo Bras Soares Passos
2021-08-31 11:02 ` [PATCH v1 2/3] io: Add zerocopy and errqueue Leonardo Bras
2021-08-31 12:57 ` Daniel P. Berrangé
2021-08-31 20:27 ` Peter Xu
2021-09-01 8:50 ` Daniel P. Berrangé
2021-09-01 15:52 ` Peter Xu
2021-09-01 15:59 ` Daniel P. Berrangé
2021-09-02 7:07 ` Leonardo Bras Soares Passos [this message]
2021-09-02 6:59 ` Leonardo Bras Soares Passos
2021-09-07 16:44 ` Peter Xu
2021-09-08 20:13 ` Leonardo Bras Soares Passos
2021-09-08 21:04 ` Peter Xu
2021-09-02 6:38 ` Leonardo Bras Soares Passos
2021-09-02 8:47 ` Daniel P. Berrangé
2021-09-02 9:34 ` Leonardo Bras Soares Passos
2021-09-02 9:49 ` Daniel P. Berrangé
2021-09-02 10:19 ` Leonardo Bras Soares Passos
2021-09-02 10:28 ` Daniel P. Berrangé
2021-09-07 11:06 ` Dr. David Alan Gilbert
2021-09-07 18:09 ` Peter Xu
2021-09-08 8:30 ` Dr. David Alan Gilbert
2021-09-08 15:24 ` Peter Xu
2021-09-09 8:49 ` Dr. David Alan Gilbert
2021-09-08 20:25 ` Leonardo Bras Soares Passos
2021-09-08 21:09 ` Peter Xu
2021-09-08 21:57 ` Daniel P. Berrangé
2021-09-09 2:05 ` Peter Xu
2021-09-09 4:58 ` Leonardo Bras Soares Passos
2021-09-09 16:40 ` Peter Xu
2021-08-31 11:02 ` [PATCH v1 3/3] migration: multifd: Enable zerocopy Leonardo Bras
2021-08-31 13:16 ` Daniel P. Berrangé
2021-08-31 20:29 ` Peter Xu
2021-09-01 8:53 ` Daniel P. Berrangé
2021-09-01 15:35 ` Peter Xu
2021-09-01 15:44 ` Daniel P. Berrangé
2021-09-01 16:01 ` Peter Xu
2021-09-02 7:57 ` Leonardo Bras Soares Passos
2021-09-07 11:13 ` Dr. David Alan Gilbert
2021-09-08 15:26 ` Daniel P. Berrangé
2021-09-02 7:23 ` Jason Wang
2021-09-02 8:08 ` Leonardo Bras Soares Passos
2021-09-02 7:27 ` Leonardo Bras Soares Passos
2021-09-02 7:22 ` Leonardo Bras Soares Passos
2021-09-02 8:20 ` Daniel P. Berrangé
2021-09-02 8:52 ` Leonardo Bras Soares Passos
2021-09-02 9:20 ` Daniel P. Berrangé
2021-09-02 9:49 ` Leonardo Bras Soares Passos
2021-09-02 9:59 ` Daniel P. Berrangé
2021-09-02 10:25 ` Leonardo Bras Soares Passos
2021-09-07 11:17 ` Dr. David Alan Gilbert
2021-09-07 18:32 ` Peter Xu
2021-09-08 2:59 ` Jason Wang
2021-09-08 3:24 ` Peter Xu
2021-09-08 3:26 ` Jason Wang
2021-09-08 8:19 ` Dr. David Alan Gilbert
2021-09-08 15:19 ` Peter Xu
2021-09-09 1:10 ` Jason Wang
2021-08-31 21:24 ` [PATCH v1 0/3] QIOChannel flags + multifd zerocopy Peter Xu
2021-09-01 19:21 ` Leonardo Bras Soares Passos
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