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: Mon, 02 Nov 2015 22:48:14 +0000 Message-ID: <1446504494.15061.66.camel@infradead.org> References: <1445789224-28032-1-git-send-email-shamir.rabinovitch@oracle.com> <1445789224-28032-2-git-send-email-shamir.rabinovitch@oracle.com> <1446013801.3405.183.camel@infradead.org> <20151028111049.GA30785@shamir-ThinkPad-T430> <1446039110.3405.212.camel@infradead.org> <1446078721.1856.49.camel@kernel.crashing.org> <1446079332.3405.273.camel@infradead.org> <1446081046.1856.55.camel@kernel.crashing.org> <1446158125.4471.5.camel@infradead.org> <20151101074534.GC23022@shamir-ThinkPad-T430> <1446412249.4060.7.camel@kernel.crashing.org> Mime-Version: 1.0 Content-Type: multipart/signed; micalg="sha-1"; protocol="application/x-pkcs7-signature"; boundary="=-pgJds/rongo6YBDgnb7M" Return-path: Received: from bombadil.infradead.org ([198.137.202.9]:44413 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751217AbbKBWs1 (ORCPT ); Mon, 2 Nov 2015 17:48:27 -0500 In-Reply-To: <1446412249.4060.7.camel@kernel.crashing.org> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Benjamin Herrenschmidt , Shamir Rabinovitch Cc: Andy Lutomirski , Christian Borntraeger , linux-arch , Paolo Bonzini , Martin Schwidefsky , "linux-doc@vger.kernel.org" , Sebastian Ott , linux-s390 , Cornelia Huck , Joerg Roedel , Jonathan Corbet , KVM , Arnd Bergmann , Christoph Hellwig --=-pgJds/rongo6YBDgnb7M Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2015-11-02 at 08:10 +1100, Benjamin Herrenschmidt wrote: > On Sun, 2015-11-01 at 09:45 +0200, Shamir Rabinovitch wrote: > > Not sure this use case is possible for Infiniband where application hol= d > > the data buffers and there is no way to force application to re use the= =20 > > buffer as suggested. > >=20 > > This is why I think there will be no easy way to bypass the DMA mapping= cost > > for all use cases unless we allow applications to request bypass/pass t= hrough > > DMA mapping (implicitly or explicitly). >=20 > But but but ... >=20 > What I don't understand is how that brings you any safety. >=20 > Basically, either your bridge has a bypass window, or it doesn't. (Or > it has one and it's enabled or not, same thing). >=20 > If you request the bypass on a per-mapping basis, you basically have to > keep the window always enabled, unless you do some nasty refcounting of > how many people have a bypass mapping in flight, but that doesn't seem > useful. >=20 > Thus you have already lost all protection from the device, since your > entire memory is accessible via the bypass mapping. >=20 > Which means what is the point of then having non-bypass mappings for > other things ? Just to make things slower ? >=20 > I really don't see what this whole "bypass on a per-mapping basis" buys > you. In the Intel case, the mapping setup is entirely per-device (except for crap devices and devices behind a PCIe-PCI bridge, etc.). So you can happily have a passthrough mapping for *one* device, without making that same mapping available to another device. You can make that trade-off of speed vs. protection for each device in the system. Currently we have the 'iommu=3Dpt' option that makes us use passthrough for *all* devices (except those whose dma_mask can't reach all of physical memory). But I could live with something like Shamir is proposing, which lets us do the bypass only for performance-sensitive devices which actually *ask* for it (and of course we'd have a system- wide mode which declines that request and does normal mappings anyway). --=20 dwmw2 --=-pgJds/rongo6YBDgnb7M 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 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