From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4EA04C433E3 for ; Wed, 15 Jul 2020 07:06:35 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 231882065E for ; Wed, 15 Jul 2020 07:06:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 231882065E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:49370 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jvbUw-0000Rr-E5 for qemu-devel@archiver.kernel.org; Wed, 15 Jul 2020 03:06:34 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:51468) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jvbUO-0008SS-LY for qemu-devel@nongnu.org; Wed, 15 Jul 2020 03:06:00 -0400 Received: from [195.135.220.15] (port=47520 helo=mx2.suse.de) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jvbUM-0007ku-US for qemu-devel@nongnu.org; Wed, 15 Jul 2020 03:06:00 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 0EC47AD78; Wed, 15 Jul 2020 07:06:00 +0000 (UTC) Message-ID: <4b9cc7d60fdbd8fa41686fb94ed55e354fe6fa20.camel@suse.com> Subject: Re: [PATCH] virtio-rng: return available data with O_NONBLOCK From: Martin Wilck To: "Michael S. Tsirkin" , Jason Wang Date: Wed, 15 Jul 2020 09:05:56 +0200 In-Reply-To: <20200714220019.10854-1-mwilck@suse.com> References: <20200714220019.10854-1-mwilck@suse.com> Content-Type: text/plain; charset="ISO-8859-15" User-Agent: Evolution 3.36.3 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Host-Lookup-Failed: Reverse DNS lookup failed for 195.135.220.15 (failed) Received-SPF: pass client-ip=195.135.220.15; envelope-from=mwilck@suse.com; helo=mx2.suse.de X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/14 23:03:40 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x (no timestamps) [generic] X-Spam_score_int: -33 X-Spam_score: -3.4 X-Spam_bar: --- X-Spam_report: (-3.4 / 5.0 requ) BAYES_00=-1.9, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, RDNS_NONE=0.793, 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: qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, 2020-07-15 at 00:00 +0200, mwilck@suse.com wrote: > From: Martin Wilck > > If a program opens /dev/hwrng with O_NONBLOCK and uses poll() and > non-blocking read() to retrieve random data, it ends up in a tight > loop with poll() always returning POLLIN and read() returning EAGAIN. > This repeats forever until some process makes a blocking read() call. > The reason is that virtio_read() always returns 0 in non-blocking > mode, > even if data is available. > > The following test program illustrates the behavior. ... > This can be observed in the real word e.g. with nested qemu/KVM > virtual > machines, if both the "outer" and "inner" VMs have a virtio-rng > device. > If the "inner" VM requests random data, qemu running in the "outer" > VM > uses this device in a non-blocking manner like the test program > above. > > Fix it by returning available data if it exists. > > Signed-off-by: Martin Wilck > --- > drivers/char/hw_random/virtio-rng.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/drivers/char/hw_random/virtio-rng.c > b/drivers/char/hw_random/virtio-rng.c > index 79a6e47b5fbc..94806308d814 100644 > --- a/drivers/char/hw_random/virtio-rng.c > +++ b/drivers/char/hw_random/virtio-rng.c > @@ -59,6 +59,9 @@ static int virtio_read(struct hwrng *rng, void > *buf, size_t size, bool wait) > if (vi->hwrng_removed) > return -ENODEV; > > + if (vi->data_avail >= size || (vi->data_avail && !wait)) > + return vi->data_avail; > + > if (!vi->busy) { > vi->busy = true; > reinit_completion(&vi->have_data); This patch was nonsense. I'm sorry. Looking into it again. Martin