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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 96C81C369AB for ; Tue, 15 Apr 2025 11:04:35 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1u4e4m-000403-Cj; Tue, 15 Apr 2025 07:03:36 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1u4e4i-0003za-Bs for qemu-devel@nongnu.org; Tue, 15 Apr 2025 07:03:33 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1u4e4f-0005O9-O3 for qemu-devel@nongnu.org; Tue, 15 Apr 2025 07:03:31 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1744715007; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=1r0Vt9SO+HOAkulgNbwubjMkc2HKq9YChoxlKi5p5CY=; b=KQv/lwkxF6fuN/obUpeKt+0mgyiQHfwdxqylCMaFy6Tey9HIeStws2HHGG2E0qDJxgQJBS Aki0U6Xgha+Vtas94d29qjyc1u7zfKgz61fJX/MvFA6stZaP63R600tGmSONHMCRF3S9wg qzXO0lgCB9nrs/nG90dUewe7TcnnN4U= Received: from mx-prod-mc-02.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-642-mHJY_LIDPW6qz5HM1mvG7g-1; Tue, 15 Apr 2025 07:03:25 -0400 X-MC-Unique: mHJY_LIDPW6qz5HM1mvG7g-1 X-Mimecast-MFC-AGG-ID: mHJY_LIDPW6qz5HM1mvG7g_1744715004 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id F0743195609F; Tue, 15 Apr 2025 11:03:23 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.133]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id D63C51801766; Tue, 15 Apr 2025 11:03:20 +0000 (UTC) Date: Tue, 15 Apr 2025 12:03:17 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Prasad Pandit Cc: Marco Cavenati , Peter Xu , Fabiano Rosas , qemu-devel@nongnu.org Subject: Re: [PATCH] migration: add FEATURE_SEEKABLE to QIOChannelBlock Message-ID: References: <20250327141451.163744-3-Marco.Cavenati@eurecom.fr> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.13 (2024-03-09) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, 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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Apr 15, 2025 at 04:14:08PM +0530, Prasad Pandit wrote: > Hi, > > On Tue, 15 Apr 2025 at 15:51, Daniel P. Berrangé wrote: > > It is actually NOT really connected to lseek, and as such > > * For file and fd channels _FEATURE_SEEKABLE is set when/if lseek(2) works. > -> https://gitlab.com/qemu-project/qemu/-/commit/401e311ff72e0a62c834bfe466de68a82cfd90cb Yes I know, checking lseek is a proxy that determines whether preadv will work or not. This is basically validating that the file/fd is not a FIFO/pipe/character device. It must be a block device or regular file. > > QIO_CHANNEL_FEATURE_SEEKABLE is probably a bad choice of name > > in retrospect. > > * That's plausible. Because while reading code, _SEEKABLE indicates > (or hints) that the underlying stream allows random access at a given > offset. Even pread(2)/pwrite(2) manuals say that -> file referenced by > fd should be capable of seeking. So correlation is that, since > QIO_CHANNEL is an abstraction layer, it supports different > streams/channels underneath, maybe some of those underneath streams > support seeking (random access) and some don't. Hence we set > _FEATURE_SEEKABLE when the underlying channel/stream supports it. > > > In QIOChanel API specification, having QIO_CHANNEL_FEATURE_SEEKABLE > > set is a pre-requisite for use of qio_channel_{pread,preadv,pwrite,pwritev}. > > * If *_FEATURE_SEEKABLE is not connected to lseek(2) or seeking, what > is its right interpretation? Why is it a pre-requisite for use of > qio_channel_{pread,preadv,pwrite,pwritev} functions? Because that's what the QEMU API specification declares * Not all implementations will support this facility, so may report * an error. To avoid errors, the caller may check for the feature * flag QIO_CHANNEL_FEATURE_SEEKABLE prior to calling this method. and what the QEMU API impl defines ssize_t qio_channel_pwritev(QIOChannel *ioc, const struct iovec *iov, size_t niov, off_t offset, Error **errp) { QIOChannelClass *klass = QIO_CHANNEL_GET_CLASS(ioc); if (!klass->io_pwritev) { error_setg(errp, "Channel does not support pwritev"); return -1; } if (!qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_SEEKABLE)) { error_setg_errno(errp, EINVAL, "Requested channel is not seekable"); return -1; } return klass->io_pwritev(ioc, iov, niov, offset, errp); } With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|