From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alex Williamson Subject: Re: VFIO no-iommu Date: Tue, 15 Dec 2015 09:53:18 -0700 Message-ID: <1450198398.6042.32.camel@redhat.com> References: <60420822.AbcfvjLZCk@xps13> <566B4A50.9090607@6wind.com> <1449874953.20509.6.camel@redhat.com> <26FA93C7ED1EAA44AB77D62FBE1D27BA6747CE55@IRSMSX108.ger.corp.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable To: "O'Driscoll, Tim" , Vincent JARDIN , "dev@dpdk.org" Return-path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by dpdk.org (Postfix) with ESMTP id DBE985921 for ; Tue, 15 Dec 2015 17:53:21 +0100 (CET) In-Reply-To: <26FA93C7ED1EAA44AB77D62FBE1D27BA6747CE55@IRSMSX108.ger.corp.intel.com> List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" On Tue, 2015-12-15 at 13:43 +0000, O'Driscoll, Tim wrote: > > -----Original Message----- > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Alex > > Williamson > > Sent: Friday, December 11, 2015 11:03 PM > > To: Vincent JARDIN; dev@dpdk.org > > Subject: Re: [dpdk-dev] VFIO no-iommu > >=20 > > On Fri, 2015-12-11 at 23:12 +0100, Vincent JARDIN wrote: > > > Thanks Thomas for putting back this topic. > > >=20 > > > Alex, > > >=20 > > > I'd like to hear more about the impacts of "unsupported": > > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/c > > > ommi > > > t/?id=3D033291eccbdb1b70ffc02641edae19ac825dc75d > > > =C2=A0=C2=A0=C2=A0Use of this mode, specifically binding a device w= ithout a > > > native > > > =C2=A0=C2=A0=C2=A0IOMMU group to a VFIO bus driver will taint the k= ernel and > > > should > > > =C2=A0=C2=A0=C2=A0therefore not be considered supported. > > >=20 > > > It means that we get ride of uio; so it is a nice code cleanup: > > > but > > > why > > > would VFIO/NO IOMMU be better if the bottomline is "unsupported"? > >=20 > > How supportable do you think the uio method is? =C2=A0Fundamentally w= e > > have > > a userspace driver doing unrestricted DMA; it can access and modify > > any > > memory in the system. =C2=A0This is the reason uio won't provide a > > mechanism > > to enable MSI and if you ask the uio maintainer, they don't support > > DMA > > at all, it's only intended as a programmed IO interface to the > > device. > > =C2=A0Unless we can sandbox a user owned device within an IOMMU > > protected > > container, it's not supportable. =C2=A0The VFIO no-iommu mode can sim= ply > > provide you that unsupported mode more easily since it leverages > > code > > from the supported mode, which is IOMMU protected. =C2=A0Thanks, >=20 > Thanks for clarifying. >=20 > This does seem like it would be useful for DPDK. We're doing some > further investigation to see if it works out of the box with DPDK or > if we need to make any changes to support it. The iommu model is different, there's no type1 interface available when using this mode since we have no ability to provide translation. =C2=A0Th= e no-iommu iommu model really does nothing, which is a possible issue for userspace. =C2=A0Is it sufficient? =C2=A0We stopped short of creating a p= age pinning interface through the no-iommu model because it requires code and adding piles of new code for an interface we claim is unsupported doesn't make a lot of sense. =C2=A0The device interface should be identic= al to existing vfio support. > Thomas highlighted that your original commit for this had been > reverted. What specifically would you need from us in order to re- > submit the VFIO No-IOMMU support? No API changes should ever go into the kernel without being validated by a user. =C2=A0Without that we're risking that the kernel interface is broken and we're stuck supporting it. =C2=A0In this case I tried to make sure we had a working user before it went it, gambled that it was close enough to put in anyway, then paid the price when development went silent on the user side. =C2=A0To get it back in, I'm going to need a working use first. =C2=A0You can re-apply 033291eccbdb or re- revert=C2=A0ae5515d66362 for development of that. =C2=A0I need to see tha= t it works and that there's some consensus from the dpdk community that it's a worthwhile path forward for cases without an iommu. =C2=A0There's no po= int in merging it if it only becomes a userspace proof of concept. =C2=A0Than= ks, Alex