From mboxrd@z Thu Jan 1 00:00:00 1970 From: Robin Murphy Subject: Re: [RFC PATCH] vfio/type1: Do not support IOMMUs that allow bypass Date: Thu, 29 Oct 2015 18:42:10 +0000 Message-ID: <56326882.10109@arm.com> References: <20151015205046.28129.50479.stgit@gimli.home> <5621039F.50609@linaro.org> <1445010682.4059.508.camel@redhat.com> <20151027154043.GF1689@arm.com> <1445961611.8018.269.camel@redhat.com> <20151029182819.GJ3440@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Cc: iommu , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, kvm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, "eric.auger" To: Will Deacon , Alex Williamson Return-path: In-Reply-To: <20151029182819.GJ3440-5wv7dgnIgG8@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 List-Id: kvm.vger.kernel.org On 29/10/15 18:28, Will Deacon wrote: > On Tue, Oct 27, 2015 at 10:00:11AM -0600, Alex Williamson wrote: >> On Tue, 2015-10-27 at 15:40 +0000, Will Deacon wrote: >>> On Fri, Oct 16, 2015 at 09:51:22AM -0600, Alex Williamson wrote: >>>> Would it be possible to add iommu_domain_geometry support to arm-smmu.c? >>>> In addition to this test to verify that DMA cannot bypass the IOMMU, I'd >>>> eventually like to pass the aperture information out through the VFIO >>>> API. Thanks, >>> >>> The slight snag here is that we don't know which SMMU in the system the >>> domain is attached to at the point when the geometry is queried, so I >>> can't give you an upper bound on the aperture. For example, if there is >>> an SMMU with a 32-bit input address and another with a 48-bit input >>> address. >>> >>> We could play the same horrible games that we do with the pgsize bitmap, >>> and truncate some global aperture everytime we probe an SMMU device, but >>> I'd really like to have fewer hacks like that if possible. The root of >>> the problem is still that domains are allocated for a bus, rather than >>> an IOMMU instance. >> >> Yes, Intel VT-d has this issue as well. In theory we can have >> heterogeneous IOMMU hardware units (DRHDs) in a system and the upper >> bound of the geometry could be diminished if we add a less capable DRHD >> into the domain. I suspect this is more a theoretical problem than a >> practical one though as we're typically mixing similar DRHDs and I think >> we're still capable of 39-bit addressing in the least capable version >> per the spec. >> >> In any case, I really want to start testing geometry.force_aperture, >> even if we're not yet comfortable to expose the IOMMU limits to the >> user. The vfio type1 shouldn't be enabled at all for underlying >> hardware that allows DMA bypass. Thanks, > > Ok, I'll put it on my list of things to look at under the assumption that > the actual aperture limits don't need to be accurate as long as DMA to > an arbitrary unmapped address always faults. I'm pretty sure we'd only ever set the aperture to the full input address range anyway (since we're not a GART-type thing), in which case we should only need to worry about unmatched streams that don't hit in a domain at all. Doesn't the disable_bypass option already cover that? (FWIW I hacked it up for v2 a while back, too[0]). Robin. [0]:http://www.linux-arm.org/git?p=linux-rm.git;a=commitdiff;h=23a251189fa3330b799a837bd8eb1023aa2dcea4