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 ED2D7192D68; Mon, 7 Apr 2025 07:54:49 +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=1744012492; cv=none; b=q5N2ZKYYwudG5uUtt8JLs7tP4Znv5xD7sVoo8yKdqVgALEumwp0BHHTkJT4p81m3Dz9HCVTtpwDygyfhoKyra8FIkiuMDEdXNljtbhzaOZbw3QkJ7RQU/tirWdMw1dJRUsuXx3oLxWrHbGeGEdCOyb1Yi8NJ0dYP4o1WDkvjOfY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1744012492; c=relaxed/simple; bh=SOZUqudDNfVBB30xlSpIXat86v1zi5Cu5tisd1gvvw0=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=XEOVNWg6OW12vgB8IHziHO8biH2zIJoCc2ov0Bv74TEWrCTKpWCFD6bxtyRi2D/1QBCKRLqsfpLriXpB/7nT3e4neWoP2djZaETyOkCtb7/vGZvBjFFGlRwjxhBuz9SSY3NErRFPKQtShhJ3kGk0ArbMBRCgQz0/FbolTBRA8Rw= 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=IHXuTen5; 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="IHXuTen5" 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=SOZUqudDNfVBB30xlSpIXat86v1zi5Cu5tisd1gvvw0=; b=IHXuTen5rUGs9EX4Y6Y0WeG6n9 s70R3Eq36Py2UYQ/bemX8p/nLBZkpoBE/PPzxRXtUQ//cpHLLCytPNF/nkamHBu3kZtF1bMkb2Yht uL/xsxJJIZc02xJcHf+J3sdJjhAxiU1trT0O8KfvP/1hmK/00OvA78Te4m0rB8T6RjKMIUBx6BEp0 kGPBpTt0Tx4zUjYhPCM4w8Ah5NDh0G2d6RNhser+NY0SE1GT2RyiknXOsWqq/ClVOFTqYyRqPB1wo EQctUfalzYA8cSwf28WIt7ZU4Ucp/G+FTjuINYCgQzMpBncrl7o5QyGtdDz8ytg5ig2KK51bxcTLE QF0N6Zyg==; Received: from [172.31.31.145] (helo=u09cd745991455d.lumleys.internal) by desiato.infradead.org with esmtpsa (Exim 4.98.1 #2 (Red Hat Linux)) id 1u1hJe-00000007sgu-3M0I; Mon, 07 Apr 2025 07:54:46 +0000 Message-ID: Subject: Re: [RFC PATCH 1/3] content: Add VIRTIO_F_SWIOTLB to negotiate use of SWIOTLB bounce buffers From: David Woodhouse To: Christoph Hellwig Cc: "Michael S. Tsirkin" , virtio-comment@lists.linux.dev, 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: Mon, 07 Apr 2025 08:54:46 +0100 In-Reply-To: References: <05abb68286dd4bc17b243130d7982a334503095b.camel@infradead.org> <20250404040838-mutt-send-email-mst@kernel.org> <67bd998bfe385088ef863342b9f8714754585476.camel@infradead.org> <20250404043016-mutt-send-email-mst@kernel.org> <5cc2f558b0f4d387349c3a2936ff00430804536d.camel@infradead.org> <20250404062409-mutt-send-email-mst@kernel.org> <7fd789b61a586417add2115f6752ebec5e7b81bf.camel@infradead.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-W3jU6GFfEbE2PIQ3ApaF" 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 --=-W3jU6GFfEbE2PIQ3ApaF Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2025-04-07 at 00:30 -0700, Christoph Hellwig wrote: > On Fri, Apr 04, 2025 at 12:15:52PM +0100, David Woodhouse wrote: > > We could achieve that by presenting the device with a completely new > > PCI device/vendor ID so that old drivers don't match, or in the DT > > model you could make a new "compatible" string for it. I chose to use a > > VIRTIO_F_ bit for it instead, which seemed natural and allows the > > device model (under the influence of the system integrator) to *choose* > > whether a failure to negotiate such bit is fatal or not. >=20 > Stop thinking about devices.=C2=A0 Your CoCo VM will have that exact same > limitation for all devices, because none of them can DMA into random > memory. Nah, most of them are just fine because they're actual passthrough PCI devices behind a proper 2-stage IOMMU. > > So on x86 it might be an e820-reserved region for example. >=20 > Hasn't e820 replaced with something more "elaborate" for UEFI systems > anyway? Sure, it's no longer literally a BIOS call with 0xe820 in any registers but Linux still calls it that. > > I don't think we want the guest OS just *assuming* that there's usable > > memory in that e820-reserved region, just because some device says that > > it's actually capable of DMA to those addresses. > >=20 > > So it would probably want a separate object, like the separate > > `restricted-dma-pool` in DT, which explicitly identifies that range as > > a DMA bounce-buffer pool. We probably *can* do that even in ACPI with a > > PRP0001 device today, using a `restricted-dma-pool` compatible > > property. > >=20 > > Then the OS would need to spot this range in the config space, and say > > "oh, I *do* have a swiotlb pool this device can reach", and use that. >=20 > Yes, that's largely how it should work. The problem in ACPI is matching the device to that SWIOTLB pool. I think we can expose a `restricted-dma-pool` node via PRP0001 but then we need to associate a particular device (or set of devices) to that pool. In DT we do that by referencing it from a `memory-region` node of the device itself. The idea above was that the affected devices would state that they are only capable of DMA to that range, and that's how we'd match them up.=20 --=-W3jU6GFfEbE2PIQ3ApaF Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCD9Aw ggSOMIIDdqADAgECAhAOmiw0ECVD4cWj5DqVrT9PMA0GCSqGSIb3DQEBCwUAMGUxCzAJBgNVBAYT AlVTMRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5jb20xJDAi BgNVBAMTG0RpZ2lDZXJ0IEFzc3VyZWQgSUQgUm9vdCBDQTAeFw0yNDAxMzAwMDAwMDBaFw0zMTEx MDkyMzU5NTlaMEExCzAJBgNVBAYTAkFVMRAwDgYDVQQKEwdWZXJva2V5MSAwHgYDVQQDExdWZXJv a2V5IFNlY3VyZSBFbWFpbCBHMjCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMjvgLKj jfhCFqxYyRiW8g3cNFAvltDbK5AzcOaR7yVzVGadr4YcCVxjKrEJOgi7WEOH8rUgCNB5cTD8N/Et GfZI+LGqSv0YtNa54T9D1AWJy08ZKkWvfGGIXN9UFAPMJ6OLLH/UUEgFa+7KlrEvMUupDFGnnR06 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