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 lists1p.gnu.org (lists1p.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 19260CD8C9D for ; Mon, 8 Jun 2026 11:39:31 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wWYJy-0005B4-HY; Mon, 08 Jun 2026 07:39:11 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wWYJw-0005AU-7s for qemu-arm@nongnu.org; Mon, 08 Jun 2026 07:39:08 -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 1wWYJt-0006YY-Ft for qemu-arm@nongnu.org; Mon, 08 Jun 2026 07:39:07 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1780918743; 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=R7U8ggQahttMFndg7bBnoA6ZA81bWpOUflLpAgHxqM4=; b=eELdQCR6oP7Sqhw9BozbrMWcKPfr8xresoqkKPFECddgJk2ygk5FVL67wPKtuel2Y2UGGZ mAhm02aDHZZxLBBweHHO0Epo0ra2QFUk9C7pbFmxDpUw/NHSKeZCrMIv2HsSZLAXqQpKeW 4b72oLbVMNBC6znIlEkCMZwd8bQ282A= Received: from mx-prod-mc-01.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-8-iwXai69yPte3MtlYnicYCw-1; Mon, 08 Jun 2026 07:39:00 -0400 X-MC-Unique: iwXai69yPte3MtlYnicYCw-1 X-Mimecast-MFC-AGG-ID: iwXai69yPte3MtlYnicYCw_1780918739 Received: from mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.95]) (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-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id C4992195609F; Mon, 8 Jun 2026 11:38:58 +0000 (UTC) Received: from redhat.com (unknown [10.44.49.219]) by mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 51E24765; Mon, 8 Jun 2026 11:38:55 +0000 (UTC) Date: Mon, 8 Jun 2026 12:38:51 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Gavin Shan Cc: Peter Xu , qemu-devel@nongnu.org, qemu-arm@nongnu.org, mst@redhat.com, jugraham@redhat.com, shan.gavin@gmail.com Subject: Re: [PATCH RFCv1] virtio: Inherit max bounce buffer size from bus parent if possible Message-ID: References: <20260608001821.850921-1-gshan@redhat.com> <07ca74b4-52a8-4187-a57c-7c3277e574d3@redhat.com> MIME-Version: 1.0 In-Reply-To: <07ca74b4-52a8-4187-a57c-7c3277e574d3@redhat.com> User-Agent: Mutt/2.3.2 (2026-04-26) X-Scanned-By: MIMEDefang 3.6 on 10.30.177.95 X-Mimecast-MFC-PROC-ID: Ulx41vkB0YqyKzqfcqdb0bk4_1t-cJq0vatlfqsuTi4_1780918739 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit 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: 8 X-Spam_score: 0.8 X-Spam_bar: / X-Spam_report: (0.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.445, 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_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-arm@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-arm-bounces+qemu-arm=archiver.kernel.org@nongnu.org Sender: qemu-arm-bounces+qemu-arm=archiver.kernel.org@nongnu.org On Mon, Jun 08, 2026 at 09:11:50PM +1000, Gavin Shan wrote: > Hi Daniel, > > On 6/8/26 6:55 PM, Daniel P. Berrangé wrote: > > On Mon, Jun 08, 2026 at 10:18:21AM +1000, Gavin Shan wrote: > > > On the guest where a NVidia's GH100 card is passed from the host, the > > > guest system hang can be observed on attempt to compile 'cuda-samples', > > > as reported by Julia. > > > > snip > > > > Thanks for looking into this. NB, I didn't really look into it beyond noticing the suggestion that users set an "x-" property as a proposed solution to failing to boot, which raised a red-flag to me from a usability POV. I don't really know anything about the underlying technical problems here, so can't offer specific guidance in that area. > > > > By default, the max bounce buffer size is only 4096 bytes, even less > > > than one page when the guest page is 64KB. This tries to fix the issue > > > by inheriting the customized max bounce buffer size of the virtio bus's > > > parent through property 'x-max-bounce-buffer-size' when the customized > > > size is a larger one. With this applied, no guest system hang is seen > > > with '-device virtio-blk-pci,...,x-max-bounce-buffer-size=268435456'. > > > > "x-max-bounce-buffer-size" is an experimental / unsupported property. > > > > We really shouldn't be expecting users to have to set this in a production > > deployment in order to stop a guest from hanging. Even if we dropped the > > experimental marker from this property, users would still need to know to > > provide this magic setting, so it would still be broken out of the box. > > > > How can we get a solution that "just works" out of the box, which is > > fully supported, not relying on experimental properties ? > > > > How do we know that "x-max-bounce-buffer-size" is an experimental or unsupported > property? I guess the properties whose names start with "x-" are all treated as > experimental and unsupported? Yes, any QEMU property starting with 'x-' is experimental/unstable/ unsupported and can be changed/withdrawn at any time. Libvirt will not provide any way to configure 'x-' properties, as it requires a supported/stable solution from QEMU. With regards, Daniel -- |: https://berrange.com ~~ https://hachyderm.io/@berrange :| |: https://libvirt.org ~~ https://entangle-photo.org :| |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :|