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

On 2020-08-26 19:21, Robin Murphy wrote:

On 2020-08-26 13:17, Sai Prakash Ranjan wrote:

quoted

On 2020-08-26 17:07, Robin Murphy wrote:

quoted

On 2020-08-25 16:42, Sai Prakash Ranjan wrote:

quoted

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/ (local) 
[3]
https://lore.kernel.org/linux-iommu/20200626002710.110200-2-rajatja@google.com/ (local)

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.

-Sai

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.

Robin.

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