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 11C04241676; Thu, 3 Apr 2025 08:10:43 +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=1743667845; cv=none; b=ZYAZvsCrw0euAOMgLCPrxTPI/SEYiiTKtqOvDXUceiXzFgApvWaLQEMTTh6zh0N7Wjk5u2zh2QTYKVADx2icCM6xeJN19ooFg7sV8M2ZpK0ULBqwmqU1FtVPE6thHgAZuRRlnAxCyODqRdkSa2hGH8tkQ67QIFD1v15OvCnsL0A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743667845; c=relaxed/simple; bh=yRINf8NCBCvDr3SI7DevhOlkyKdNKV40uEvwq7n8GdM=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=EJPTWCRyNp/2JlxlmTcaTO3W+b/LIlDrXbydSLitWVz7wgiOdxQQrlHi+solUyWdqnednpyHxVDo2EuRWe8FPdoPCZICKcu/bcqjTvAYqZeUIQ9PdFhyOw0xDTfqK0BsdffLTrbg0y+nQg3ePX1AXv0jSVI8xgthAJhnM8kCv7I= 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=UG2NET9Z; 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="UG2NET9Z" 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=yRINf8NCBCvDr3SI7DevhOlkyKdNKV40uEvwq7n8GdM=; b=UG2NET9ZYcw3/6fx5VAYk9Zi8m id36Zki5XeFV+nNtjCZAcaCD35et/Lhz2D+O9smCk5qC8U4ikdFBh6XReMFoIW51JIdUJnEHUEXIf 1EbyqDKAoaJNr+lOmmFikSZnJJ27P/K9mfYfbS8WKEqIhnCM1ufp2g4PEmASQJNQd6O9NwOUzZMmf GVRVD/ZJnLwlL7OSbAFr7kQ38kNPIs6MMsVSBMLlRH2TSPFnFf9gdsC8aJfx1X3ED9Ps8BEILbi9D lBh84FrneIpW4X8Lvlw5mxE9VknIptG7AR9FAAiJVfNA81L3SB29qvnM2Qj+TFIt7XS+E/hHlLxTc /De0MYPg==; 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 1u0Fer-000000079al-2CJq; Thu, 03 Apr 2025 08:10:41 +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: Thu, 03 Apr 2025 09:10:41 +0100 In-Reply-To: References: <20250402105137-mutt-send-email-mst@kernel.org> <19ba662feeb93157bc8a03fb0b11cb5f2eca5e40.camel@infradead.org> <20250402111901-mutt-send-email-mst@kernel.org> <6b3b047f1650d91abe5e523dd7f862c6f7ee6611.camel@infradead.org> <20250402114757-mutt-send-email-mst@kernel.org> <965ccf2f972c5d5f1f4edacb227f03171f20e887.camel@infradead.org> <20250402124131-mutt-send-email-mst@kernel.org> <06465bcf4422d088df2a0ce9cdb09767dac83118.camel@infradead.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-mv6TI6TyS2mihHsGmBJj" 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 --=-mv6TI6TyS2mihHsGmBJj Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2025-04-03 at 00:39 -0700, Christoph Hellwig wrote: > On Thu, Apr 03, 2025 at 08:37:20AM +0100, David Woodhouse wrote: > > Hm. I was just trying to point out what seemed obvious, that when a PCI > > device does 'DMA' to an address region which is actually within one of > > its *own* BARs, >=20 > PCIe devices can't do DMA to their own BARs by definition, see the route > to self rule. >=20 > Pretending that they do it by parsing the addresses is bound to fail > because the addresses seen by the driver and the device can be > different. >=20 > NVMe got this wrong not just once but twice and is still suffering from > this misunderstanding.=C2=A0 If you want to enhance a protocol to support > addressing a local indirection buffer do not treat it as fake DMA > but rather use explicit addressing for it, or you will be in a world of > trouble. This is, of course, the other benefit of pointing out the "obvious". Because you can get corrected when you've got it wrong :) Thanks. I'll take a closer look at handling that. I think it's reasonable for the negotiation of the VIRTIO_F_SWIOTLB feature to be the thing that switches *all* addresses to be on-device, and the on- device buffer can't be accessed unless VIRTIO_F_SWIOTLB has been negotiated. Which neatly sidesteps the original thing I was trying to clarify anyway. --=-mv6TI6TyS2mihHsGmBJj 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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