From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8BA411E511; Wed, 2 Apr 2025 15:47:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743608846; cv=none; b=oAENwFEEqMLmZFJlG0aKNq8bkXu+/GQZRTeI/+TlS6YfL4+XK6MY91cyifMyMUaI+vzkwAgNYoz8KO0+p/HcJOli/oj1MNd19fC6KNG56k760/3w2hysBrZyOk2gkFLz3+Bq7Mh2R+LJGAXOQbgK44ER9w4vSVmwib84ftBHeA4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743608846; c=relaxed/simple; bh=ianagpQrzxuiEeJ8qW2ygl6eJKVkOCgieWnMU/rRnEQ=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=Ia2I7kPQzcTqquKVF7VII+QuDQ6KbtpMC/1W8Qq2nJOm67tioHbwakVL8bwZIe8GX2rTuEbl8hERGd2xv8qQ5M8T9v6lo7vFkFp7/LX8y+Y+Ffr0docIOvlKTkS9pPIQEjlfFgeDM8jhYNR4r+5Y0Cq+GA7/d0JM2uCyc0ncLX4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=desiato.srs.infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=bVeSAzRj; arc=none smtp.client-ip=90.155.92.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=desiato.srs.infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="bVeSAzRj" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=MIME-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=ianagpQrzxuiEeJ8qW2ygl6eJKVkOCgieWnMU/rRnEQ=; b=bVeSAzRj0JOrev4EhTfFjc6TTp AARQ5+7ADehtDROhy/uOTlzl2uIHPsvcAfezfo7iyeW5bsyMvXq2G4IKBVIm72Y7hmd/o/Z1JU6jV Y4xhIooYaq3f7YWsis8vpP8D8BsiLB8KlY1wQcSagVTsxUs60OOYSgEjLa9FfQyEWz9nqn3Q1dwbC BdSTIzQ2LVtLnt7L0xGVSrUN+SosDKOc/G+tkxzJ/tyjWFYBFj27KMQar6m0y3812Ra+b6FVce7+7 3iwgJpjtoflzGedwO+gIJd6rQh4bKwvK/7BZLlZToMh1wKXY1j+/pP6tPRss89//Eh1w7QfmvWirh ppXFu/tA==; Received: from [172.31.31.145] (helo=u09cd745991455d.ant.amazon.com) by desiato.infradead.org with esmtpsa (Exim 4.98.1 #2 (Red Hat Linux)) id 1u00JC-000000073sF-2Y13; Wed, 02 Apr 2025 15:47:18 +0000 Message-ID: <6b3b047f1650d91abe5e523dd7f862c6f7ee6611.camel@infradead.org> Subject: Re: [RFC PATCH 1/3] content: Add VIRTIO_F_SWIOTLB to negotiate use of SWIOTLB bounce buffers From: David Woodhouse To: "Michael S. Tsirkin" Cc: virtio-comment@lists.linux.dev, hch@infradead.org, Claire Chang , linux-devicetree , Rob Herring , =?ISO-8859-1?Q?J=F6rg?= Roedel , iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, graf@amazon.de Date: Wed, 02 Apr 2025 16:47:18 +0100 In-Reply-To: <20250402111901-mutt-send-email-mst@kernel.org> References: <20250402112410.2086892-1-dwmw2@infradead.org> <20250402112410.2086892-2-dwmw2@infradead.org> <20250402105137-mutt-send-email-mst@kernel.org> <19ba662feeb93157bc8a03fb0b11cb5f2eca5e40.camel@infradead.org> <20250402111901-mutt-send-email-mst@kernel.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-6RjDHDqdoAIfGBoxa/uv" User-Agent: Evolution 3.52.3-0ubuntu1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by desiato.infradead.org. See http://www.infradead.org/rpr.html --=-6RjDHDqdoAIfGBoxa/uv Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, 2025-04-02 at 11:20 -0400, Michael S. Tsirkin wrote: > On Wed, Apr 02, 2025 at 04:12:39PM +0100, David Woodhouse wrote: > > On Wed, 2025-04-02 at 10:54 -0400, Michael S. Tsirkin wrote: > > > > +=C2=A0 If a the device transport provides a software IOTLB bounce = buffer, > > > > +=C2=A0 addresses within its range are not subject to the requireme= nts of > > > > +=C2=A0 VIRTIO_F_ACCESS_PLATFORM as they are considered to be ``on-= device''. > > >=20 > > > I don't get this part. the system designers currently have a choice > > > whether to have these controlled by VIRTIO_F_ACCESS_PLATFORM or not. > > > with PCI, for example, BAR on the same device is naturally not > > > behind an iommu. > >=20 > > In the PCI case this *is* a BAR on the same device, and is naturally > > not behind an IOMMU as you say. This is just stating the obvious, for > > clarity. >=20 > Then the platform already does this right, and it's better not to > try and override it in the spec. It isn't intended as an "override". This *is* what will happen if the platform does it right. By mandating it in the spec, the intent is to reduce the chances of platforms doing it *wrong*? (Or of drivers making wrong assumptions). > > For virtio-mmio it also isn't translated by an IOMMU; that was the > > *point* of the `restricted-dma-pool` support. > >=20 >=20 > Clear VIRTIO_F_ACCESS_PLATFORM then? > I don't want to say that VIRTIO_F_ACCESS_PLATFORM and VIRTIO_F_SWIOTLB > are mutually exclusive... > Generally, it is preferable to keep all features orthogonal if > at all possible. ...precisely because they *should* be orthogonal. VIRTIO_F_ACCESS_PLATFORM defines how system memory is accessed; basically whether DMA goes through an IOMMU or not. And as you point out, the "on-device" buffer used with VIRTIO_F_SWIOTLB=C2=A0should never pass through the IOMMU anyway, so it *is* naturally orthogonal. And I think it does make sense for both to be set in some cases, for both physical and virtual devices. For physical devices that would mean "Got an IOMMU? Sure, go ahead and use it. If not, if you don't trust me, you can just disable my bus mastering and just use the SWIOTLB". It's basically the same for a virtual device. In a confidential compute model, the device model (in the VMM) might not be *able* to access the guest memory unless the core guest OS explicitly permits that, through some kind of pKVM enlightenment to allow pages to be shared, or a vIOMMU, or marking hardware pages unencrypted. So having both bits set would mean "Know how to drive that enlightenment? Sure, go ahead and use it. Otherwise, use the SWIOTLB". In fact that's exactly what that Linux code for `restricted-dma-pool` already does =E2=80=94 when setting up the dma_ops for the device, if it fi= nds an actual set of IOMMU operations, it'll use those. 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