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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6A09CC433FE for ; Mon, 4 Oct 2021 14:33:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 52D2A61002 for ; Mon, 4 Oct 2021 14:33:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234498AbhJDOf1 (ORCPT ); Mon, 4 Oct 2021 10:35:27 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:37638 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234290AbhJDOfY (ORCPT ); Mon, 4 Oct 2021 10:35:24 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1633358014; 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=tkFXR+Ar045yOW1Qn+SywsZzwu6VhSEtfZbFfXfqIlo=; b=hsliw9iCbTaOj2zpc3io4iIwZpay5GEFAFdYBp0B8hiPbDfXMGwpLgRTp7D4xriOPPisQb c65yBda+C4eh2nPBq8qJ97v6na/SgXyWLlbV/LTtcy1P+Aul3H2Tdrts3DByAGl+++RyAJ vVBO+XkKK+3KogdqCaduz5Ko9ht0nkM= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-324-nNKntssOPQqiw5sEur7mCA-1; Mon, 04 Oct 2021 10:33:31 -0400 X-MC-Unique: nNKntssOPQqiw5sEur7mCA-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 4893B835DE2; Mon, 4 Oct 2021 14:33:30 +0000 (UTC) Received: from localhost (unknown [10.39.193.66]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B7DA0100238C; Mon, 4 Oct 2021 14:33:22 +0000 (UTC) From: Cornelia Huck To: "Michael S. Tsirkin" Cc: Halil Pasic , Jason Wang , Xie Yongji , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, markver@us.ibm.com, Christian Borntraeger , linux-s390@vger.kernel.org, qemu-devel@nongnu.org Subject: Re: [RFC PATCH 1/1] virtio: write back features before verify In-Reply-To: <20211004090018-mutt-send-email-mst@kernel.org> Organization: Red Hat GmbH References: <20210930012049.3780865-1-pasic@linux.ibm.com> <20210930070444-mutt-send-email-mst@kernel.org> <20211001092125.64fef348.pasic@linux.ibm.com> <20211002055605-mutt-send-email-mst@kernel.org> <87bl452d90.fsf@redhat.com> <20211004090018-mutt-send-email-mst@kernel.org> User-Agent: Notmuch/0.32.1 (https://notmuchmail.org) Date: Mon, 04 Oct 2021 16:33:21 +0200 Message-ID: <875yuc3ln2.fsf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 04 2021, "Michael S. Tsirkin" wrote: > On Mon, Oct 04, 2021 at 02:19:55PM +0200, Cornelia Huck wrote: >> >> [cc:qemu-devel] >> >> On Sat, Oct 02 2021, "Michael S. Tsirkin" wrote: >> >> > On Fri, Oct 01, 2021 at 09:21:25AM +0200, Halil Pasic wrote: >> >> On Thu, 30 Sep 2021 07:12:21 -0400 >> >> "Michael S. Tsirkin" wrote: >> >> >> >> > On Thu, Sep 30, 2021 at 03:20:49AM +0200, Halil Pasic wrote: >> >> > > This patch fixes a regression introduced by commit 82e89ea077b9 >> >> > > ("virtio-blk: Add validation for block size in config space") and >> >> > > enables similar checks in verify() on big endian platforms. >> >> > > >> >> > > The problem with checking multi-byte config fields in the verify >> >> > > callback, on big endian platforms, and with a possibly transitional >> >> > > device is the following. The verify() callback is called between >> >> > > config->get_features() and virtio_finalize_features(). That we have a >> >> > > device that offered F_VERSION_1 then we have the following options >> >> > > either the device is transitional, and then it has to present the legacy >> >> > > interface, i.e. a big endian config space until F_VERSION_1 is >> >> > > negotiated, or we have a non-transitional device, which makes >> >> > > F_VERSION_1 mandatory, and only implements the non-legacy interface and >> >> > > thus presents a little endian config space. Because at this point we >> >> > > can't know if the device is transitional or non-transitional, we can't >> >> > > know do we need to byte swap or not. >> >> > >> >> > Hmm which transport does this refer to? >> >> >> >> It is the same with virtio-ccw and virtio-pci. I see the same problem >> >> with both on s390x. I didn't try with virtio-blk-pci-non-transitional >> >> yet (have to figure out how to do that with libvirt) for pci I used >> >> virtio-blk-pci. >> >> >> >> > Distinguishing between legacy and modern drivers is transport >> >> > specific. PCI presents >> >> > legacy and modern at separate addresses so distinguishing >> >> > between these two should be no trouble. >> >> >> >> You mean the device id? Yes that is bolted down in the spec, but >> >> currently we don't exploit that information. Furthermore there >> >> is a fat chance that with QEMU even the allegedly non-transitional >> >> devices only present a little endian config space after VERSION_1 >> >> was negotiated. Namely get_config for virtio-blk is implemented in >> >> virtio_blk_update_config() which does virtio_stl_p(vdev, >> >> &blkcfg.blk_size, blk_size) and in there we don't care >> >> about transitional or not: >> >> >> >> static inline bool virtio_access_is_big_endian(VirtIODevice *vdev) >> >> { >> >> #if defined(LEGACY_VIRTIO_IS_BIENDIAN) >> >> return virtio_is_big_endian(vdev); >> >> #elif defined(TARGET_WORDS_BIGENDIAN) >> >> if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) { >> >> /* Devices conforming to VIRTIO 1.0 or later are always LE. */ >> >> return false; >> >> } >> >> return true; >> >> #else >> >> return false; >> >> #endif >> >> } >> >> >> > >> > ok so that's a QEMU bug. Any virtio 1.0 and up >> > compatible device must use LE. >> > It can also present a legacy config space where the >> > endian depends on the guest. >> >> So, how is the virtio core supposed to determine this? A >> transport-specific callback? > > I'd say a field in VirtIODevice is easiest. The transport needs to set this as soon as it has figured out whether we're using legacy or not. I guess we also need to fence off any accesses respectively error out the device if the driver tries any read/write operations that would depend on that knowledge? And using a field in VirtIODevice would probably need some care when migrating. Hm...