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Wed, 4 Mar 2020 19:47:12 +0000 (UTC) Received: from work-vm (ovpn-116-225.ams2.redhat.com [10.36.116.225]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E006D19C69; Wed, 4 Mar 2020 19:46:59 +0000 (UTC) Date: Wed, 4 Mar 2020 19:46:57 +0000 From: "Dr. David Alan Gilbert" To: Jag Raman Subject: Re: [PATCH v5 08/50] multi-process: add functions to synchronize proxy and remote endpoints Message-ID: <20200304194657.GL4104@work-vm> References: <3d013f0091ac6babb55f30cadf6ed806c322b7ce.1582576372.git.jag.raman@oracle.com> <20200303195627.GW3170@work-vm> <8205fd32-2c74-f599-a630-1c31828cf61e@oracle.com> MIME-Version: 1.0 In-Reply-To: <8205fd32-2c74-f599-a630-1c31828cf61e@oracle.com> User-Agent: Mutt/1.13.3 (2020-01-12) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 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@oracle.com, fam@euphon.net, swapnil.ingle@nutanix.com, john.g.johnson@oracle.com, qemu-devel@nongnu.org, kraxel@redhat.com, quintela@redhat.com, mst@redhat.com, armbru@redhat.com, kanth.ghatraju@oracle.com, felipe@nutanix.com, thuth@redhat.com, ehabkost@redhat.com, konrad.wilk@oracle.com, liran.alon@oracle.com, stefanha@redhat.com, thanos.makatos@nutanix.com, rth@twiddle.net, kwolf@redhat.com, berrange@redhat.com, mreitz@redhat.com, ross.lagerwall@citrix.com, marcandre.lureau@gmail.com, pbonzini@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Jag Raman (jag.raman@oracle.com) wrote: >=20 >=20 > On 3/3/2020 2:56 PM, Dr. David Alan Gilbert wrote: > > * Jagannathan Raman (jag.raman@oracle.com) wrote: > > > In some cases, for example MMIO read, QEMU has to wait for the remote= to > > > complete a command before proceeding. An eventfd based mechanism is > > > added to synchronize QEMU & remote process. > > >=20 > > > Signed-off-by: John G Johnson > > > Signed-off-by: Jagannathan Raman > > > Signed-off-by: Elena Ufimtseva > > > --- > > > include/io/mpqemu-link.h | 7 +++++++ > > > io/mpqemu-link.c | 41 +++++++++++++++++++++++++++++++++++++= ++++ > > > 2 files changed, 48 insertions(+) > > >=20 > > > diff --git a/include/io/mpqemu-link.h b/include/io/mpqemu-link.h > > > index 2f2dd83..ae04fca 100644 > > > --- a/include/io/mpqemu-link.h > > > +++ b/include/io/mpqemu-link.h > > > @@ -135,4 +135,11 @@ void mpqemu_link_set_callback(MPQemuLinkState *s= , > > > mpqemu_link_callback callback); > > > void mpqemu_start_coms(MPQemuLinkState *s); > > > +#define GET_REMOTE_WAIT eventfd(0, EFD_CLOEXEC) > > > +#define PUT_REMOTE_WAIT(wait) close(wait) > > > +#define PROXY_LINK_WAIT_DONE 1 > > > + > > > +uint64_t wait_for_remote(int efd); > > > +void notify_proxy(int fd, uint64_t val); > > > + > > > #endif > > > diff --git a/io/mpqemu-link.c b/io/mpqemu-link.c > > > index bac120b..73b7032 100644 > > > --- a/io/mpqemu-link.c > > > +++ b/io/mpqemu-link.c > > > @@ -10,6 +10,7 @@ > > > #include "qemu/osdep.h" > > > #include "qemu-common.h" > > > +#include > > > #include "qemu/module.h" > > > #include "io/mpqemu-link.h" > > > @@ -216,6 +217,46 @@ int mpqemu_msg_recv(MPQemuMsg *msg, MPQemuChanne= l *chan) > > > return rc; > > > } > > > +uint64_t wait_for_remote(int efd) > > > +{ > > > + struct pollfd pfd =3D { .fd =3D efd, .events =3D POLLIN }; > > > + uint64_t val; > > > + int ret; > > > + > > > + ret =3D poll(&pfd, 1, 1000); > > > + > > > + switch (ret) { > > > + case 0: > > > + qemu_log_mask(LOG_REMOTE_DEBUG, "Error wait_for_remote: Time= d out\n"); > > > + /* TODO: Kick-off error recovery */ > > > + return ULLONG_MAX; > >=20 > > Shouldn't these be UINT64_MAX? >=20 > Sure, we'll change these to UINT64_MAX. >=20 > >=20 > > > + case -1: > > > + qemu_log_mask(LOG_REMOTE_DEBUG, "Poll error wait_for_remote:= %s\n", > > > + strerror(errno)); > > > + return ULLONG_MAX; > > > + default: > > > + if (read(efd, &val, sizeof(val)) =3D=3D -1) { > > > + qemu_log_mask(LOG_REMOTE_DEBUG, "Error wait_for_remote: = %s\n", > > > + strerror(errno)); > > > + return ULLONG_MAX; > > > + } > > > + } > > > + > > > + val =3D (val =3D=3D ULLONG_MAX) ? val : (val - 1); > >=20 > > Can you explain what's going on there?? >=20 > This wait_for_remote() function serves two purposes. This first one is > to synchronize QEMU and the remote process. Secondly, it allows the > remote process to return a 64-bit value to QEMU. When the remote process > has completed the task, it uses the notify_proxy() function to return to > QEMU along with a return value. >=20 > We have implemented this synchronization mechanism using eventfd, as > it's easy to setup. So the remote process could write a non-zero value > to the eventfd to wake QEMU up. However, the drawback of using eventfd > for this purpose is that a return value of zero wouldn't wake QEMU up. > Therefore, we offset the return value by one at the remote process and > correct it in the QEMU end. OK, that needs a big hairy comment to explain what's going on. Dave > -- > Jag >=20 > >=20 > > > + return val; > > > +} > > > + > > > +void notify_proxy(int efd, uint64_t val) > > > +{ > > > + val =3D (val =3D=3D ULLONG_MAX) ? val : (val + 1); > > > + > > > + if (write(efd, &val, sizeof(val)) =3D=3D -1) { > >=20 > > I'd actually check the write/read's are returning sizeof(val) - they > > can on a bad day return 0 or send only a few bytes; in theory? > >=20 > > Dave > >=20 > > > + qemu_log_mask(LOG_REMOTE_DEBUG, "Error notify_proxy: %s\n", > > > + strerror(errno)); > > > + } > > > +} > > > + > > > static gboolean mpqemu_link_handler_prepare(GSource *gsrc, gint *ti= meout) > > > { > > > g_assert(timeout); > > > --=20 > > > 1.8.3.1 > > >=20 > > -- > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > >=20 >=20 -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK