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

Doug Anderson
2020-08-26T15:07:51Z

Hi,

On Wed, Aug 26, 2020 at 8:01 AM Sai Prakash Ranjan
<saiprakash.ranjan@codeaurora.org> wrote:
>
> On 2020-08-26 19:21, Robin Murphy wrote:
> > On 2020-08-26 13:17, Sai Prakash Ranjan wrote:
> >> On 2020-08-26 17:07, Robin Murphy wrote:
> >>> On 2020-08-25 16:42, Sai Prakash Ranjan 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".
> >>>
> >>> There seems to be considerable overlap here with both the existing
> >>> patches for per-device default domain control [1], and the broader
> >>> ongoing development on how to define, evaluate and handle "trusted"
> >>> vs. "untrusted" devices (e.g. [2],[3]). I'd rather see work done to
> >>> make sure those integrate properly together and work well for
> >>> everyone's purposes, than add more disjoint mechanisms that only
> >>> address small pieces of the overall issue.
> >>>
> >>> Robin.
> >>>
> >>> [1]
> >>> https://lore.kernel.org/linux-iommu/20200824051726.7xaJRTTszJuzdFWGJ8YNsshCtfNR0BNeMrlILAyqt_0@z/
> >>> [2]
> >>> https://lore.kernel.org/linux-iommu/20200630044943.3425049-1-rajatja@google.com/
> >>> [3]
> >>> https://lore.kernel.org/linux-iommu/20200626002710.110200-2-rajatja@google.com/
> >>
> >> Thanks for the links, [1] definitely sounds interesting, I was under
> >> the impression
> >> that changing such via sysfs is late, but seems like other Sai has got
> >> it working
> >> for the default domain type. So we can extend that and add a strict
> >> attribute as well,
> >> we should be definitely OK with system booting with default strict
> >> mode for all
> >> peripherals as long as we have an option to change that later, Doug?
> >
> > Right, IIRC there was initially a proposal of a command line option
> > there too, and it faced the same criticism around not being very
> > generic or scalable. I believe sysfs works as a reasonable compromise
> > since in many cases it can be tweaked relatively early from an initrd,
> > and non-essential devices can effectively be switched at any time by
> > removing and reprobing their driver.
> >
>
> Ah I see, so the catch is that device must not be bound to the driver
> and won't work for the internal devices or builtin drivers probed early.

Hrm, that wouldn't work so well for us for eMMC. I don't think I'm
going to manage to convince folks that we need an initrd just for
this. I'm probably being naive and I haven't looked at the code, but
it does seem a little weird that this isn't the kind of thing that
could just be tweaked for transfers going forward...

> > As for a general approach for internal devices where you do believe
> > the hardware is honest but don't necessarily trust whatever firmware
> > it happens to be running, I'm pretty sure that's come up already, but
> > I'll be sure to mention it at Rajat's imminent LPC talk if nobody else
> > does.

I'll at least attend. We'll see how useful my contributions are
since, as per usual, I'm wandering into an area I'm not an expert in
here. ;-)

-Doug