From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
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 DD0E4C7EE23
	for <linux-kernel@archiver.kernel.org>; Fri, 19 May 2023 12:42:22 +0000 (UTC)
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S230460AbjESMmV (ORCPT <rfc822;linux-kernel@archiver.kernel.org>);
        Fri, 19 May 2023 08:42:21 -0400
Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56554 "EHLO
        lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S231336AbjESMmS (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 19 May 2023 08:42:18 -0400
Received: from verein.lst.de (verein.lst.de [213.95.11.211])
        by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3640B10D
        for <linux-kernel@vger.kernel.org>; Fri, 19 May 2023 05:42:00 -0700 (PDT)
Received: by verein.lst.de (Postfix, from userid 2407)
        id A400368C7B; Fri, 19 May 2023 14:41:18 +0200 (CEST)
Date:   Fri, 19 May 2023 14:41:18 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Marek =?iso-8859-1?Q?Marczykowski-G=F3recki?= 
        <marmarek@invisiblethingslab.com>
Cc:     Christoph Hellwig <hch@lst.de>, Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, Ben Skeggs <bskeggs@redhat.com>,
        Karol Herbst <kherbst@redhat.com>,
        Lyude Paul <lyude@redhat.com>, xen-devel@lists.xenproject.org,
        iommu@lists.linux.dev, linux-kernel@vger.kernel.org,
        nouveau@lists.freedesktop.org
Subject: Re: [PATCH 2/4] x86: always initialize xen-swiotlb when
 xen-pcifront is enabling
Message-ID: <20230519124118.GA5869@lst.de>
References: <20230518134253.909623-1-hch@lst.de> <20230518134253.909623-3-hch@lst.de> <ZGZr/xgbUmVqpOpN@mail-itl> <20230519040405.GA10818@lst.de> <ZGdLErBzi9MANL3i@mail-itl>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <ZGdLErBzi9MANL3i@mail-itl>
User-Agent: Mutt/1.5.17 (2007-11-01)
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, May 19, 2023 at 12:10:26PM +0200, Marek Marczykowski-Górecki wrote:
> While I would say PCI passthrough is not very common for PV guests, can
> the decision about xen-swiotlb be delayed until you can enumerate
> xenstore to check if there are any PCI devices connected (and not
> allocate xen-swiotlb by default if there are none)? This would
> still not cover the hotplug case (in which case, you'd need to force it
> with a cmdline), but at least you wouldn't loose much memory just
> because one of your VMs may use PCI passthrough (so, you have it enabled
> in your kernel).

How early can we query xenstore?  We'd need to do this before setting
up DMA for any device.

The alternative would be to finally merge swiotlb-xen into swiotlb, in
which case we might be able to do this later.  Let me see what I can
do there.