Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 22 Mar 2022 10:16:26 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     joro@8bytes.org, baolu.lu@linux.intel.com,
        andreas.noever@gmail.com, michael.jamet@intel.com,
        mika.westerberg@linux.intel.com, YehezkelShB@gmail.com,
        iommu@lists.linux-foundation.org, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org, mario.limonciello@amd.com, hch@lst.de
Subject: Re: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection more
 accurate
Message-ID: <20220322091626.GB27069@lst.de>
References: <cover.1647624084.git.robin.murphy@arm.com> <0dd14883930c9f55ace22162e23765a37d91a057.1647624084.git.robin.murphy@arm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <0dd14883930c9f55ace22162e23765a37d91a057.1647624084.git.robin.murphy@arm.com>
User-Agent: Mutt/1.5.17 (2007-11-01)
Precedence: bulk

On Fri, Mar 18, 2022 at 05:42:58PM +0000, Robin Murphy wrote:
> Between me trying to get rid of iommu_present() and Mario wanting to
> support the AMD equivalent of DMAR_PLATFORM_OPT_IN, scrutiny has shown
> that the iommu_dma_protection attribute is being far too optimistic.
> Even if an IOMMU might be present for some PCI segment in the system,
> that doesn't necessarily mean it provides translation for the device(s)
> we care about. Furthermore, all that DMAR_PLATFORM_OPT_IN really does
> is tell us that memory was protected before the kernel was loaded, and
> prevent the user from disabling the intel-iommu driver entirely. While
> that lets us assume kernel integrity, what matters for actual runtime
> DMA protection is whether we trust individual devices, based on the
> "external facing" property that we expect firmware to describe for
> Thunderbolt ports.
> 
> It's proven challenging to determine the appropriate ports accurately
> given the variety of possible topologies, so while still not getting a
> perfect answer, by putting enough faith in firmware we can at least get
> a good bit closer. If we can see that any device near a Thunderbolt NHI
> has all the requisites for Kernel DMA Protection, chances are that it
> *is* a relevant port, but moreover that implies that firmware is playing
> the game overall, so we'll use that to assume that all Thunderbolt ports
> should be correctly marked and thus will end up fully protected.
> 
> CC: Mario Limonciello <mario.limonciello@amd.com>
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>

Looks sensible to me:

Acked-by: Christoph Hellwig <hch@lst.de>