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 3F1B225776; Fri, 4 Apr 2025 07:50:51 +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=1743753058; cv=none; b=JtA5csda3WqDKytNnmS8OEbiFskBcAU2AiWXKsb+SHxeKmCGBnhKKKrsmWquAW3pol+ie4z/XbvuuX/Y7VZST7inTMWXkJ/v4Z55OE0pDdMdLWXBnOcc1ilSyhxdEv6a9UV4Z+sdSi6A+KZIvHRs5i9Ubbk84+dFkVXzDEQBzoA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743753058; c=relaxed/simple; bh=c0dvOM0N+ARzGWQqrSEEFwqAlK+wEPXMtlh/JFBKeck=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=qRQQ3SkDhTRR6mj2cFv9C9Vuf7/ys0/rgPstzvMqH50UaHjsXlVgaG1uk9vz5jN6bUfYqnM91+q/m3EiPqKdmzU01ugnpRuwdjfByQn9ZexQWK2iE1+N7S2cPTgNIOfG4r4oyjheqV3TSmcsq13pwQj8NOHGws3G0NppHn2ShwA= 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=KjPigiEv; 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="KjPigiEv" 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=c0dvOM0N+ARzGWQqrSEEFwqAlK+wEPXMtlh/JFBKeck=; b=KjPigiEvbWpzh2iFeUn4c4gFvA YHruz6/IMQjhKETfMLDEWSUR2JBS8MfmJsscup9TdYaPBv4HIkV7EOhjRcl05fl4sOWE6E+nobD/+ RAwzdNB4pYe2AB6f8tmUQMlq+8NZU6d15+pye500gwzAAFhwH7u3OJmJ0Rx44Lm0xGb7f/M312eN5 1DilM/dnF3cgvyy1sNOACEHCIgTkrTmhnqSQ3Np70vTvlY6OuEM5N4l2jVAwwdvsXxuHZOF2drIor cUK5lJiaTd0sa1Sj5fU3nF7XgpQH1Y8EaZwIs+NP3UIcz8/ygJhMQ9V+hKeOhQ1C0uMHAfVcPXTzq 0+jdNBuw==; 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 1u0bp9-00000007JUf-2Bx3; Fri, 04 Apr 2025 07:50:47 +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: virtio-comment@lists.linux.dev, mst@redhat.com, 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: Fri, 04 Apr 2025 08:50:47 +0100 In-Reply-To: References: <20250402112410.2086892-1-dwmw2@infradead.org> <20250402112410.2086892-2-dwmw2@infradead.org> <148a3c8ee53af585b42ec025c2c7821ad852c66c.camel@infradead.org> <05abb68286dd4bc17b243130d7982a334503095b.camel@infradead.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-kZgq5TD4LPJazVsD9Zzc" 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 --=-kZgq5TD4LPJazVsD9Zzc Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2025-04-03 at 23:35 -0700, Christoph Hellwig wrote: > As stated above I suspect you still are asking the wrong question and > have the wrong mental model.=C2=A0 Virtio fundamentally can do DMA, you j= ust > don't want it to.=C2=A0 And based on the other subthread I also suspect y= ou'd > actually be much better off with your bounce buffer in the virtual host > memory instead of coming up with this particularly odd case, but even > if not it's the system description that matters here, not the device > model. I do agree, this is fundamentally a system issue. In a CoCo model, it's non-trivial for the system to allow *virtual* devices to do "DMA" because that actually means allowing the VMM to access arbitrary guest memory. And most hardware designers screwed up the 2-stage IOMMU which allows protection from *real* devices, and didn't build in a way for the VMM to access guest memory through a stage1 mapping. Fixing that to allow system DMA to those guest devices in a way which will work across operating systems is impractical, as it involves the guest knowing which PCI device is implemented where, and either core kernel (share/unshare) enlightenments, or emulating a full IOMMU in the *trusted* part of the hypervisor (e.g. pKVM). So "for the emulated devices, just use a device model that doesn't do arbitrary DMA to system memory" is a nice simple answer, and keeps the guest support restricted to its *own* standalone driver. And yes, virtio is kind of fundamentally DMA-based as you said, but that's OK if the DMA is only to a specific range which is *known* to be shareable and which the guest would never consider to be 'normal RAM'. Because then the VMM can be *allowed* to access that. What's annoying is that this should work out of the box *already* with virtio-mmio and a `restricted-dma-pool` =E2=80=94 for systems which aren't afflicted by UEFI/ACPI/PCI as their discovery mechanisms. In fact, I may even try offering virtio-mmio in a PRP0001 ACPI device to see how that works, although I think I'll lament the lack of MSI interrupts. But the cleaner solution seemed (at least when I started this discussion) to be to add the same kind of facility to the virtio-pci transport too. And I still don't think I've quite given up on it; especially as you point out the use case for staged P2P DMA too. I'll go take a closer look at how we can cleanly disambiguate between address spaces. --=-kZgq5TD4LPJazVsD9Zzc 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 aDJAwtycb8yXtILj+TvfhLFhafxroXrflspavejQkEiHjNjtHnwbZ+o43g0/yxjwnarGI3kgcak7 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