linux-kernel - Re: [RFC/RFT] Add noats flag to boot parameters

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Message-ID: <20180503225254.GC15790@bhelgaas-glaptop.roam.corp.google.com>
Date:   Thu, 3 May 2018 17:52:54 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Sinan Kaya <okaya@...eaurora.org>
Cc:     Joerg Roedel <joro@...tes.org>, Gil Kupfer <gilkup@...il.com>,
        dwmw2@...radead.org, bhelgaas@...gle.com,
        iommu@...ts.linux-foundation.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, nadav.amit@...il.com,
        Gil Kupfer <gilkup@...technion.ac.il>,
        Will Deacon <will.deacon@....com>
Subject: Re: [RFC/RFT] Add noats flag to boot parameters

On Thu, May 03, 2018 at 10:23:02AM -0400, Sinan Kaya wrote:
> +Bjorn,
> 
> On 5/3/2018 9:59 AM, Joerg Roedel wrote:
> > On Thu, May 03, 2018 at 09:46:34AM -0400, Sinan Kaya wrote:
> >> I also like the idea in general.
> >> Minor nit..
> >>
> >> Shouldn't this be an iommu parameter rather than a PCI kernel command line parameter?
> >> We now have an iommu.passthrough argument that prevents page translation.
> >>
> >> Doesn't this fit into the same category especially when it is the IOMMU drivers that
> >> call ATS functions for enablement not the PCI drivers.
> > 
> > ATS is a bit of a grey area between PCI and IOMMU, but since ATS is
> > PCI-specific and the code to enable/disable it is in PCI as well, I
> > think the parameter makes sense for PCI too.
> 
> OK. Bjorn was interested in having a command line driven feature enables
> in driver/pci directory with bitmasks for each optional PCI spec
> capability rather than noXYZ feature.

It's true that I try to avoid adding *any* kernel parameters as much
as possible because they're usually not practical for end-users.

I think it's unreasonable to expect users to use "pci=" parameters
based on what specific hardware they have.  That's too hard to
discover and too hard to use.  I did wonder about a "pci=safe"
parameter that would disable potentially risky features just as a
debugging feature [1].

This ATS case is a security question and the parameter is not
something that would have to be used to get certain hardware to work,
so I think it's probably reasonable to add.  I would maybe expand the
documentation so it includes the reason somebody might want it, i.e.,
to defend against malicious PCIe devices.

A parameter using bitmasks could be conceivable for developers but
sounds too unwieldy for end-users.

[1] https://bugzilla.kernel.org/show_bug.cgi?id=196197#c53