From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by passt.top (Postfix) with ESMTP id 8AFA65A005E for ; Thu, 17 Nov 2022 17:03:05 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1668700984; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=wS6Z8jyycxgeHQUcOqcl6j/b50FIfJVDnFfi1Ne3qW4=; b=F3KsBvyAkDyq/nxaLj79p1txlDXc8vkC96jBIaAS2A11auW+hUZ+W2D/k+gtOElTljYA8d KIc5nfMlzqTZ0N242e8XTmysWVPSZ+TKxs3+gQ6Hevxb4C52WxSibiGHEty+FvaJgfEwwf qjy1Beb58Tfht9UvWRJvT1QS5Gj+pBQ= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-633-jO-3ecD0MyyoSZZ3Wjvy-A-1; Thu, 17 Nov 2022 11:02:59 -0500 X-MC-Unique: jO-3ecD0MyyoSZZ3Wjvy-A-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 4229B94AB12 for ; Thu, 17 Nov 2022 16:02:46 +0000 (UTC) Received: from localhost (unknown [10.39.193.198]) by smtp.corp.redhat.com (Postfix) with ESMTP id E0F6A111DCE0; Thu, 17 Nov 2022 16:02:45 +0000 (UTC) Date: Thu, 17 Nov 2022 16:02:43 +0000 From: "Richard W.M. Jones" To: Stefano Brivio Subject: Re: [PATCH v2 3/5] passt, tap: Add --fd option Message-ID: <20221117160243.GH7636@redhat.com> References: <20221117122614.1269214-4-rjones@redhat.com> <20221117152640.2535159-1-sbrivio@redhat.com> <20221117153306.GF7636@redhat.com> <20221117164931.5585f60f@elisabeth> MIME-Version: 1.0 In-Reply-To: <20221117164931.5585f60f@elisabeth> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Message-ID-Hash: 7VVYC4AC5VVT3VT2V2QC4ISHGQAJRBSW X-Message-ID-Hash: 7VVYC4AC5VVT3VT2V2QC4ISHGQAJRBSW X-MailFrom: rjones@redhat.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: passt-dev@passt.top X-Mailman-Version: 3.3.3 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: On Thu, Nov 17, 2022 at 04:49:31PM +0100, Stefano Brivio wrote: > On Thu, 17 Nov 2022 15:33:06 +0000 > "Richard W.M. Jones" wrote: > > > On Thu, Nov 17, 2022 at 04:26:40PM +0100, Stefano Brivio wrote: > > > From: "Richard W.M. Jones" > > > > > > This passes a fully connected stream socket to passt. > > > > > > Signed-off-by: Richard W.M. Jones > > > [sbrivio: reuse fd_tap instead of adding a new descriptor, > > > imply --one-off on --fd, add to optstring and usage()] > > > Signed-off-by: Stefano Brivio > > > --- > > > v2: > > > - reuse fd_tap, we don't need a separate file descriptor > > > - add F to optstring and usage(), for both passt and pasta > > > - imply --one-off, we can't do much once the socket is closed > > > > > > With this, the trick from 5/5 goes a bit further: passt reads > > > from the file descriptor passed by the wrapper. > > > > Thanks for the v2 .. I'll add it to my series and play with it. > > > > > However, we get EPOLLRDHUP right away, from the close() on > > > one end of the socket pair I guess. Should we just ignore > > > all EPOLLRDHUP events, just the first one...? > > > > Does it see the event out-of-band or does it get an in-band > > read(2) == 0 after finishing reading the data? > > Out-of-band, so to speak: we won't even recv() if we get EPOLLRDHUP > (that's handled in tap_handler()). If I do this on top of this patch: > > --- a/tap.c > +++ b/tap.c > @@ -1073,7 +1073,7 @@ void tap_sock_init(struct ctx *c) > struct epoll_event ev = { 0 }; > > ev.data.fd = c->fd_tap; > - ev.events = EPOLLIN | EPOLLET | EPOLLRDHUP; > + ev.events = EPOLLIN | EPOLLET; > > Then it gets those four bytes: > > [pid 2538704] epoll_wait(5, 0x7ffedc4a6320, 8, 1000) = 1 > [pid 2538704] recvfrom(4, 0x560797677000, 8323069, MSG_DONTWAIT, NULL, NULL) = 4 > [pid 2538704] epoll_wait(5, [], 8, 1000) = 0 > [pid 2538704] epoll_wait(5, 0x7ffedc4a6320, 8, 1000) = -1 EINTR (Interrupted system call) > > and does nothing with them, as expected. Two epoll_wait() calls later, > the syscall is interrupted, I'm not sure why and how we should react (in > main(), passt.c) in that case. With EPOLLRDHUP removed it's a bit deadlock-y. One case I commonly see is: child (qemu): 1295637 write(5, "\0\0\0\0", 4 1295637 <... write resumed>) = 4 1295637 poll([{fd=5, events=POLLIN|POLLOUT}], 1, -1 1295637 <... poll resumed>) = 1 ([{fd=5, revents=POLLOUT}]) 1295637 read(3, 1295637 <... read resumed>"", 512) = 0 1295637 shutdown(5, SHUT_WR 1295637 <... shutdown resumed>) = 0 1295637 poll([{fd=5, events=POLLIN}], 1, -1 then later the parent (passt): 1295636 epoll_wait(5, 0x7ffe93a894e0, 8, 1000) = 1 1295636 recvfrom(4, 0x5576383cf000, 8323069, MSG_DONTWAIT, NULL, NULL) = 4 1295636 epoll_wait(5, [], 8, 1000) = 0 1295636 epoll_wait(5, [], 8, 1000) = 0 1295636 epoll_wait(5, [], 8, 1000) = 0 1295636 epoll_wait(5, [], 8, 1000) = 0 1295636 epoll_wait(5, [], 8, 1000) = 0 1295636 epoll_wait(5, [], 8, 1000) = 0 1295636 epoll_wait(5, [], 8, 1000) = 0 1295636 epoll_wait(5, [], 8, 1000) = 0 1295636 epoll_wait(5, [], 8, 1000) = 0 (forever) Removing EPOLLET (edge triggered) delivers the EPOLLIN event over and over again to passt as expected: 1299436 recvfrom(4, "", 8323069, MSG_DONTWAIT, NULL, NULL) = 0 1299436 epoll_wait(5, 0x7ffdd8a65640, 8, 1000) = 1 1299436 recvfrom(4, "", 8323069, MSG_DONTWAIT, NULL, NULL) = 0 1299436 epoll_wait(5, 0x7ffdd8a65640, 8, 1000) = 1 1299436 recvfrom(4, "", 8323069, MSG_DONTWAIT, NULL, NULL) = 0 (forever) but I expected that passt would exit the first time it reads 0 from the socket. tap_handler_passt() seems like it only considers the n<0 (error) and n>0 (data) cases. What do you think about checking for n == 0 and exiting immediately if c->one_off is true? This seems to only happen under strace. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org