Re: [PATCH] iommu: Add support to filter non-strict/lazy mode based on device names

Rob Clark
2020-08-25T22:54:08Z

On Tue, Aug 25, 2020 at 3:23 PM Doug Anderson <dianders@chromium.org> wrote:
>
> Hi,
>
> On Tue, Aug 25, 2020 at 12:01 PM Sai Prakash Ranjan
> <saiprakash.ranjan@codeaurora.org> wrote:
> >
> > Hi,
> >
> > On 2020-08-25 21:40, Doug Anderson wrote:
> > > Hi,
> > >
> > > On Tue, Aug 25, 2020 at 8:43 AM Sai Prakash Ranjan
> > > <saiprakash.ranjan@codeaurora.org> wrote:
> > >>
> > >> Currently the non-strict or lazy mode of TLB invalidation can only be
> > >> set
> > >> for all or no domains. This works well for development platforms where
> > >> setting to non-strict/lazy mode is fine for performance reasons but on
> > >> production devices, we need a more fine grained control to allow only
> > >> certain peripherals to support this mode where we can be sure that it
> > >> is
> > >> safe. So add support to filter non-strict/lazy mode based on the
> > >> device
> > >> names that are passed via cmdline parameter "iommu.nonstrict_device".
> > >>
> > >> Example:
> > >> iommu.nonstrict_device="7c4000.sdhci,a600000.dwc3,6048000.etr"
>
> Just curious: are device names like this really guaranteed to be
> stable across versions?
>
>
> > > I have an inherent dislike of jamming things like this onto the
> > > command line. IMHO the command line is the last resort for specifying
> > > configuration and generally should be limited to some specialized
> > > debug options and cases where the person running the kernel needs to
> > > override a config that was set by the person (or company) compiling
> > > the kernel. Specifically, having a long/unwieldy command line makes
> > > it harder to use for the case when an end user actually wants to use
> > > it to override something. It's also just another place to look for
> > > config.
> > >
> >
> > Good thing about command line parameters are that they are optional,
> > they do
> > not specify any default behaviour (I mean they are not mandatory to be
> > set
> > for the system to be functional), so I would like to view it as an
> > optional
> > config. And this command line parameter (nonstrict_device) is strictly
> > optional
> > with default being strict already set in the driver.
> >
> > They can be passed from the bootloader via chosen node for DT platforms
> > or choose
> > a new *bootconfig* as a way to pass the cmdline but finally it does boil
> > down to
> > just another config.
>
> Never looked at bootconfig. Unfortunately it seems to require
> initramfs so that pretty much means it's out for my usage. :(
>
>
> > I agree with general boolean or single value command line parameters
> > being just
> > more messy which could just be Kconfigs instead but for multiple value
> > parameters
> > like these do not fit in Kconfig.
> >
> > As you might already know, command line also gives an advantage to the
> > end user
> > to configure system without building kernel, for this specific command
> > line its
> > very useful because the performance bump is quite noticeable when the
> > iommu.strict
> > is off. Now for end user who would not be interested in building entire
> > kernel(majority)
> > and just cares about good speeds or throughput can find this very
> > beneficial.
> > I am not talking about one specific OS usecase here but more in general
> > term.
> >
> > > The other problem is that this doesn't necessarily scale very well.
> > > While it works OK for embedded cases it doesn't work terribly well for
> > > distributions. I know that in an out-of-band thread you indicated
> > > that it doesn't break anything that's not already broken (AKA this
> > > doesn't fix the distro case but it doesn't make it worse), it would be
> > > better to come up with a more universal solution.
> > >
> >
> > Is the universal solution here referring to fix all the command line
> > parameters
> > in the kernel or this specific command line? Are we going to remove any
> > more
> > addition to the cmdline ;)
>
> There are very few cases where a kernel command line parameter is the
> only way to configure something. Most of the time it's just there to
> override a config. I wouldn't suggest removing those. I just don't
> want a kernel command line parameter to be the primary way to enable
> something.
>
>
> > So possible other solution is the *bootconfig* which is again just
> > another place
> > to look for a config. So thing is that this universal solution would
> > result in
> > just more new fancy ways of passing configs or adding such configs to
> > the drivers
> > or subsystems in kernel which is pretty much similar to implementing
> > policy in
> > kernel which I think is frowned upon and mentioned in the other thread.
> >
> > > Ideally it feels like we should figure out how to tag devices in a
> > > generic manner automatically (hardcode at the driver or in the device
> > > tree). I think the out-of-band discussions talked about "external
> > > facing" and the like. We could also, perhaps, tag devices that have
> > > "binary blob" firmware if we wanted. Then we'd have a policy (set by
> > > Kconfig, perhaps overridable via commandline) that indicated the
> > > strictness level for the various classes of devices. So policy would
> > > be decided by KConfig and/or command line.
> > >
> >
> > How is tagging in driver or device tree better than the simple command
> > line
> > approach to pass the same list of devices which otherwise you would
> > hardcode
> > in the corresponding drivers and device tree all over the kernel other
> > than
> > the scalability part for command line? IMHO it is too much churn.
>
> It's better because it doesn't require keeping track and updating
> these per-board (or per machine) arguments for each and every
> board/machine you maintain. If, for instance, we start out by
> allowing HW video decoder to use non-strict. So:
>
> On one board, we add in "aa00000.video-codec" to the command line.
> On some other board, maybe we add in "1d00000.video-codec" to the command line.
> On some other board, maybe we add in "90400000.video-codec" to the command line.
>
> Now we realize that there's some problem and we have to remove it, so
> we need to go through and remove this from our command line
> everywhere. Worse is that we have to proactively notice it and remove
> it.
>
> Instead, let's imagine that we set a policy at a bit of a higher
> level. Different ideas:
>
> a) We could have a CONFIG_ option for the video codec that's something
> like "CONFIG_VIDEOCODEC_DEFAULT_NONSTRICT". If this was set then if
> there is no "iommu.strict" command line argument then this device
> would be mapped as non-strict. If "iommu.strict=0" or
> "iommu.strict=1" is on the command line then it would override all of
> these defaults. Probably the existence (or maybe the default value)
> of this CONFIG option implies that there are no known/expected
> exploits related to it.

jumping in a bit late so might have missed some of the background, but
shouldn't the driver decide strict vs non-strict? It should be the
one that knows if non-strict is safe or not?

BR,
-R

>
> b) We could find some way to tag the video codec and then set
> non-strictness on certain classes of devices, then we could have a
> policy to disable strictness on certain classes of devices.
>
> The nice thing about the above is that you could imagine someone
> pushing a change to the stable trees that would fix everyone affected.
> Nobody would need to go around and adjust command line options, they'd
> just get the newest stable and it could cause devices to move into
> strict mode if there was a known exploit. I suppose with your
> proposal stable trees could have a "blacklist" where the commandline
> is ignored for exploited devices, but that seems ugly.
>
>
> > Device tree could be used but then we have a problem with it being for
> > only
> > describing hardware and it doesn't work for ACPI based systems.
> >
> > Command line approach works for all systems (both DT and ACPI) without
> > having
> > to add too much churn to drivers. Lastly, I think we can have both
> > options, it
> > doesn't hurt to add command line parameter since it is optional.
>
> I'm not opposed to something existing that lets you override this on
> the command line, but I'm just not a fan of it being the primary way.
>
> -Doug
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