linux-kernel - Re: [PATCH] thunderbolt: Stop using iommu

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Message-ID: <YjIb+XOGZbWKpQDa@lahna>
Date:   Wed, 16 Mar 2022 19:18:49 +0200
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     andreas.noever@...il.com, michael.jamet@...el.com,
        YehezkelShB@...il.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        mario.limonciello@....com, hch@....de
Subject: Re: [PATCH] thunderbolt: Stop using iommu_present()

Hi,

On Wed, Mar 16, 2022 at 02:49:09PM +0000, Robin Murphy wrote:
> > What we want is to make sure the Tunneled PCIe ports get the full IOMMU
> > protection. In case of the discrete above it is also fine if all the
> > devices behind the PCIe root port get the full IOMMU protection. Note in
> > the integrated all the devices are "siblings".
> 
> Ah, OK, I wasn't aware that the NHI isn't even the right thing in the first
> place :(
> 
> Is there an easy way to get from the struct tb to a PCI device representing
> the end of its relevant tunnel, or do we have a circular dependency problem
> where the latter won't appear until we've authorised it (and thus the IOMMU
> layer won't know about it yet either)?

The PCIe root ports (and the PCIe downstream ports) are there already
even without "authorization".

There is a way to figure out the "tunneled" PCIe ports by looking at
certain properties and we do that already actually. The BIOS has the
following under these ports:

https://docs.microsoft.com/en-us/windows-hardware/drivers/pci/dsd-for-pcie-root-ports#identifying-externally-exposed-pcie-root-ports

and the ports will have dev->external_facing set to 1. Perhaps looking
at that field helps here?