From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: [PATCH] iommu/intel: Exclude devices using RMRRs from IOMMU API domains Date: Wed, 21 May 2014 11:38:36 +0100 Message-ID: <1400668716.13839.66.camel@shinybook.infradead.org> References: <20140514192620.7767.43842.stgit@bling.home> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============8344968420862966025==" Return-path: In-Reply-To: <20140514192620.7767.43842.stgit-xdHQ/5r00wBBDLzU/O5InQ@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Alex Williamson Cc: iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, chegu_vinod-VXdhtT5mjnY@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: iommu@lists.linux-foundation.org --===============8344968420862966025== Content-Type: multipart/signed; micalg="sha-1"; protocol="application/x-pkcs7-signature"; boundary="=-JAPnpTPANKKjFzQT5C7e" --=-JAPnpTPANKKjFzQT5C7e Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, 2014-05-14 at 13:27 -0600, Alex Williamson wrote: > The user of the IOMMU API domain expects to have full control of > the IOVA space for the domain. RMRRs are fundamentally incompatible > with that idea. We can neither map the RMRR into the IOMMU API > domain, nor can we guarantee that the device won't continue DMA with > the area described by the RMRR as part of the new domain. Therefore > we must prevent such devices from being used by the IOMMU API. Ick, ick, ick. The more the ramifications of RMRRs become apparent, the more I wish we'd just done the Right Thing=E2=84=A2 and declared that firmw= are SHALL NOT leave any device doing (IOMMU-visible) DMA after the OS takes over. That way, if they wanted this kind of abomination then they'd have to come up with a way of doing it differently. Hell, can't you do PCIe transactions which claim to be already translated, and thus just bypass the IOMMU? OK, rant over...=20 Why can't we map the RMRR into the IOMMU API domain? If we're setting up a VM guest, that basically means we'd want to poke a hole in its memory map and mark the RMRR-afflicted range as reserved or absent. It's horrible, but *everything* about RMRRs is horrible. It's not impossible, and it would allow us to give these devices away to guests. Don't we sometimes *have* devices that we want to give to guests, that are afflicted with RMRRs? There are discussions about RMRRs being (ab)used for more than their existing brain-damaged purpose. Where we have a peripheral device that will (mis)interpret certain address ranges as "local" rather than forwarding transactions up towards main memory, we need to ensure that such ranges are never used as virtual addresses. This has largely been an invisible problem until we found a device where the affected range matched the IOVA our DMA API uses by default. Using an RMRR has been proposed as a simple way to achieve that... which means that you end up not being able to assign *those* devices to IOMMU domains either. I do suspect it's going to lead to complaints... but I'm just not sure I can bring myself to care. Sane designs don't require RMRRs. If someone comes to me and complains that their HP storage controller or whatever can't be assigned to a guest, I'm quite prepared to tell them to replace it with something non-broken. Will you back me up when it happens? --=20 dwmw2 --=-JAPnpTPANKKjFzQT5C7e Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAQAAoIISxDCCBjQw ggQcoAMCAQICAR4wDQYJKoZIhvcNAQEFBQAwfTELMAkGA1UEBhMCSUwxFjAUBgNVBAoTDVN0YXJ0 Q29tIEx0ZC4xKzApBgNVBAsTIlNlY3VyZSBEaWdpdGFsIENlcnRpZmljYXRlIFNpZ25pbmcxKTAn BgNVBAMTIFN0YXJ0Q29tIENlcnRpZmljYXRpb24gQXV0aG9yaXR5MB4XDTA3MTAyNDIxMDE1NVoX DTE3MTAyNDIxMDE1NVowgYwxCzAJBgNVBAYTAklMMRYwFAYDVQQKEw1TdGFydENvbSBMdGQuMSsw KQYDVQQLEyJTZWN1cmUgRGlnaXRhbCBDZXJ0aWZpY2F0ZSBTaWduaW5nMTgwNgYDVQQDEy9TdGFy dENvbSBDbGFzcyAxIFByaW1hcnkgSW50ZXJtZWRpYXRlIENsaWVudCBDQTCCASIwDQYJKoZIhvcN AQEBBQADggEPADCCAQoCggEBAMcJg8zOLdgasSmkLhOrlr6KMoOMpohBllVHrdRvEg/q6r8jR+EK 75xCGhR8ToREoqe7zM9/UnC6TS2y9UKTpT1v7RSMzR0t6ndl0TWBuUr/UXBhPk+Kmy7bI4yW4urC +y7P3/1/X7U8ocb8VpH/Clt+4iq7nirMcNh6qJR+xjOhV+VHzQMALuGYn5KZmc1NbJQYclsGkDxD 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