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

[Sai Prakash Ranjan]

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@xxxxxxxxxx/ 
> > > [3]
> > > https://lore.kernel.org/linux-iommu/20200626002710.110200-2-rajatja@xxxxxxxxxx/
> >
> > 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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