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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7C4D3C433F5 for ; Sun, 2 Oct 2022 09:00:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229544AbiJBJAA (ORCPT ); Sun, 2 Oct 2022 05:00:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35160 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229509AbiJBI74 (ORCPT ); Sun, 2 Oct 2022 04:59:56 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1AAC51400E for ; Sun, 2 Oct 2022 01:59:51 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1664701190; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=uUV7VJSlnQzJ38mxDgh5xaYxjeTtv351HITh3SQqr0M=; b=J46bbuSI5IceDu/XUYlcFprpldDrc2at2L3QOgyjjgZ0nrMk6A5QbB/rZPK1zCErmaqPEu 8IcM0MqzIDsTt5EWehSnuP8S/TyQZtNr64Pek5+phLy+M5KrR7Xj/PyhxDrH+SDnl8EBHL QNOA9ttI82hclbPucAYNbpd1SMrSlMI= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-614-A5meftXDO-6l48yy-yhurA-1; Sun, 02 Oct 2022 04:59:47 -0400 X-MC-Unique: A5meftXDO-6l48yy-yhurA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 26727380670A; Sun, 2 Oct 2022 08:59:47 +0000 (UTC) Received: from starship (unknown [10.40.192.10]) by smtp.corp.redhat.com (Postfix) with ESMTP id CD77B40C206B; Sun, 2 Oct 2022 08:59:44 +0000 (UTC) Message-ID: <28ce86c01271c1b9b8f96a7783b55a8d458325d2.camel@redhat.com> Subject: Re: Commit 'iomap: add support for dma aligned direct-io' causes qemu/KVM boot failures From: Maxim Levitsky To: Paolo Bonzini , Christoph Hellwig , Keith Busch Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, qemu-devel@nongnu.org, kvm@vger.kernel.org, Kevin Wolf , Michael Roth Date: Sun, 02 Oct 2022 11:59:42 +0300 In-Reply-To: <32db4f89-a83f-aac4-5d27-0801bdca60bf@redhat.com> References: <20220929163931.GA10232@lst.de> <32db4f89-a83f-aac4-5d27-0801bdca60bf@redhat.com> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.36.5 (3.36.5-2.fc32) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2022-09-29 at 19:35 +0200, Paolo Bonzini wrote: > On 9/29/22 18:39, Christoph Hellwig wrote: > > On Thu, Sep 29, 2022 at 10:37:22AM -0600, Keith Busch wrote: > > > > I am aware, and I've submitted the fix to qemu here: > > > > > > > > https://lists.nongnu.org/archive/html/qemu-block/2022-09/msg00398.html > > > > > > I don't think so. Memory alignment and length granularity are two completely > > > different concepts. If anything, the kernel's ABI had been that the length > > > requirement was also required for the memory alignment, not the other way > > > around. That usage will continue working with this kernel patch. Yes, this is how I also understand it - for example for O_DIRECT on a file which resides on 4K block device, you have to use page aligned buffers. But here after the patch, 512 aligned buffer starts working as well - If I understand you correctly the ABI didn't guarantee that such usage would fail, but rather that it might fail. > > > > Well, Linus does treat anything that breaks significant userspace > > as a regression. Qemu certainly is significant, but that might depend > > on bit how common configurations hitting this issue are. > > Seeing the QEMU patch, I agree that it's a QEMU bug though. I'm > surprised it has ever worked. > > It requires 4K sectors in the host but not in the guest, and can be > worked around (if not migrating) by disabling O_DIRECT. I think it's > not that awful, but we probably should do some extra releases of QEMU > stable branches. > > Paolo > I must admit I am out of the loop on the exact requirements of the O_DIRECT. If I understand that correctly, after the patch in question, qemu is able to use just 512 bytes aligned buffer to read a single 4K block from the disk, which supposed to fail but wasn't guarnteed to fail. Later qemu it submits iovec which also reads a 4K block but in two parts, and if I understand that correctly, each part (iov) is considered to be a separate IO operation, and thus each has to be in my case 4K in size, and its memory buffer *should* also be 4K aligned. (but it can work with smaller alignement as well). Assuming that I understand all of this correctly, I agree with Paolo that this is qemu bug, but I do fear that it can cause quite some problems for users, especially for users that use outdated qemu version. It might be too much to ask, but maybe add a Kconfig option to keep legacy behavier for those that need it? Best regards, Maxim Levitsky