From patchwork Wed Apr 19 17:01:35 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Andre Przywara X-Patchwork-Id: 13217153 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id DB784C6FD18 for ; Wed, 19 Apr 2023 17:01:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233079AbjDSRBz (ORCPT ); Wed, 19 Apr 2023 13:01:55 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56798 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232779AbjDSRBx (ORCPT ); Wed, 19 Apr 2023 13:01:53 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 911E86A5B for ; Wed, 19 Apr 2023 10:01:52 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 22CEE152B; Wed, 19 Apr 2023 10:02:36 -0700 (PDT) Received: from donnerap.arm.com (donnerap.cambridge.arm.com [10.1.197.42]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5BF803F5A1; Wed, 19 Apr 2023 10:01:51 -0700 (PDT) From: Andre Przywara To: Will Deacon , Julien Thierry Cc: kvm@vger.kernel.org, Alexandru Elisei , Sami Mujawar , Jean-Philippe Brucker , Marc Zyngier Subject: [PATCH kvmtool v2 1/2] virtio/rng: switch to using /dev/urandom Date: Wed, 19 Apr 2023 18:01:35 +0100 Message-Id: <20230419170136.1883584-2-andre.przywara@arm.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20230419170136.1883584-1-andre.przywara@arm.com> References: <20230419170136.1883584-1-andre.przywara@arm.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org At the moment we use /dev/random as the backing device to provide random numbers to our virtio-rng implementation. The downside of doing so is that it may block indefinitely - or return EAGAIN repeatedly in our case. On one headless system without ample noise sources (no keyboard, mouse, or network traffic) I measured 30 seconds to gain one byte of randomness. At the moment EDK II insists in waiting for all of the requsted random bytes (for its EFI_RNG_PROTOCOL runtime service) to arrive, that held up a Linux kernel boot for more than 10 minutes(!). According to the Internet(TM), on Linux /dev/urandom provides the same quality random numbers as /dev/random, it just does not block when the entropy estimation algorithm suggests so. For all practical purposes the recommendation is to just use /dev/urandom, QEMU did the switch as well in 2019 [1]. Use /dev/urandom instead of /dev/random when opening the file descriptor providing the randomness source for the virtio/rng implementation. Due to a special behaviour documented on the urandom(4) manpage, a read from /dev/urandom will never block, so we can drop the O_NONBLOCK flag. [1] https://gitlab.com/qemu-project/qemu/-/commit/a2230bd778d8 Signed-off-by: Andre Przywara Reviewed-by: Jean-Philippe Brucker --- virtio/rng.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/virtio/rng.c b/virtio/rng.c index 8f85d5ec1..e6e70ced3 100644 --- a/virtio/rng.c +++ b/virtio/rng.c @@ -166,7 +166,7 @@ int virtio_rng__init(struct kvm *kvm) if (rdev == NULL) return -ENOMEM; - rdev->fd = open("/dev/random", O_RDONLY | O_NONBLOCK); + rdev->fd = open("/dev/urandom", O_RDONLY); if (rdev->fd < 0) { r = rdev->fd; goto cleanup;