[RFC PATCH 00/10] Add Fujitsu A64FX soc entry/hardware barrier driver

[Mark Rutland]

On Fri, Jan 08, 2021 at 03:23:23PM +0100, Arnd Bergmann wrote:
> On Fri, Jan 8, 2021 at 1:54 PM Mark Rutland <mark.rutland at arm.com> wrote:
> > As a general rule, we avoid the use of IMPLEMENTATION DEFINED features
> > in Linux, as they pose a number of correctness/safety challenges and
> > come with a potentially significan long term maintenance burden that is
> > generally not justified by the features themselves. For example, such
> > features are not usable under virtualization (where a hypervisor may set
> > HCR_EL2.TIDCP, or fail to context-switch state that it is unaware of).
> 
> I am somewhat less concerned about the feature being implementation
> defined than I am about adding a custom user interface for one
> platform.

I completely agree that adding a custom interface that's platform
specific is undesireable.

> In the end, anything outside of the CPU core that ends up in a SoC
> is implementation defined, and this is usually not a problem as long
> as we have an abstraction in the kernel that hides the details from
> the user, and the system is still functional if the implementation is
> turned off for whatever reason.

I think that peripherals and other bits out in the SoC are quite
different to things built into the CPU, where there's inevitably most
significant and subtle interactions with the architecture, and can be so
closely coupled as to not have a good point to apply abstraction. We
have common ways of abstracting storage devices, but the same is not as
true for userspace instructions.

Thanks,
Mark.