From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: [PATCH v1 2/2] dma-mapping-common: add DMA attribute - DMA_ATTR_IOMMU_BYPASS Date: Wed, 28 Oct 2015 22:57:12 +0900 Message-ID: <1446040632.3405.222.camel@infradead.org> References: <1446013801.3405.183.camel@infradead.org> <20151028111049.GA30785@shamir-ThinkPad-T430> <1446039110.3405.212.camel@infradead.org> <20151028.070705.1277125569024626755.davem@davemloft.net> Mime-Version: 1.0 Content-Type: multipart/signed; micalg="sha-1"; protocol="application/x-pkcs7-signature"; boundary="=-z9yOlZyiozL9R9oBcARC" Return-path: In-Reply-To: <20151028.070705.1277125569024626755.davem@davemloft.net> Sender: kvm-owner@vger.kernel.org List-Archive: List-Post: To: David Miller Cc: shamir.rabinovitch@oracle.com, arnd@arndb.de, corbet@lwn.net, linux-doc@vger.kernel.org, linux-arch@vger.kernel.org, luto@kernel.org, jroedel@suse.de, borntraeger@de.ibm.com, cornelia.huck@de.ibm.com, sebott@linux.vnet.ibm.com, pbonzini@redhat.com, hch@lst.de, benh@kernel.crashing.org, kvm@vger.kernel.org, schwidefsky@de.ibm.com, linux-s390@vger.kernel.org List-ID: --=-z9yOlZyiozL9R9oBcARC Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, 2015-10-28 at 07:07 -0700, David Miller wrote: > In the sparc64 case, the 64-bit DMA address space is divided into > IOMMU translated and non-IOMMU translated. >=20 > You just set the high bits differently depending upon what you want. Wait, does that mean a (rogue) device could *always* get full access to physical memory just by setting the high bits appropriately? That mapping is *always* available? > So a device could use both IOMMU translated and bypass accesses at=20 > the same time. While seemingly interesting, I do not recommend we=20 > provide this kind of flexibility in our DMA interfaces. Now I could understand this if the answer to my question above was 'no'. We absolutely want the *security* all the time, and we don't want the device to be able to do stupid stuff. But if the answer was 'yes' then we take the map/unmap performance hit for... *what* benefit? On Intel we have the passthrough as an *option* and I have the same initial reaction =E2=80=94 "Hell no, we want the security". But I concede t= he performance motivation for it, and I'm not *dead* set against permitting it. If I tolerate a per-device request for passthrough mode, that might prevent people from disabling the IOMMU or putting it into passthrough mode *entirely*. So actually, I'm *improving* security... I think it makes sense to allow performance sensitive device drivers to *request* a passthrough mode. The platform can reserve the right to refuse, if either the IOMMU hardware doesn't support that, or we're in a paranoid mode (with iommu=3Dalways or something on the command line). --=20 dwmw2 --=-z9yOlZyiozL9R9oBcARC Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAQAAoIISjjCCBicw ggUPoAMCAQICAw3vNzANBgkqhkiG9w0BAQUFADCBjDELMAkGA1UEBhMCSUwxFjAUBgNVBAoTDVN0 YXJ0Q29tIEx0ZC4xKzApBgNVBAsTIlNlY3VyZSBEaWdpdGFsIENlcnRpZmljYXRlIFNpZ25pbmcx ODA2BgNVBAMTL1N0YXJ0Q29tIENsYXNzIDEgUHJpbWFyeSBJbnRlcm1lZGlhdGUgQ2xpZW50IENB MB4XDTE1MDUwNTA5NDM0MVoXDTE2MDUwNTA5NTMzNlowQjEcMBoGA1UEAwwTZHdtdzJAaW5mcmFk ZWFkLm9yZzEiMCAGCSqGSIb3DQEJARYTZHdtdzJAaW5mcmFkZWFkLm9yZzCCASIwDQYJKoZIhvcN AQEBBQADggEPADCCAQoCggEBAMkbm9kPbx1j/X4RVyf/pPKSYwelcco69TvnQQbKM8m8xkWjXJI1 jpJ1jMaGUZGFToINMSZi7lZawUozudWbXSKy1SikENSTJHffsdRAIlsp+hR8vWvjsKUry6sEdqPG 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