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

[misono.tomohiro at fujitsu.com]

> On Fri, Jan 08, 2021 at 07:52:31PM +0900, Misono Tomohiro wrote:
> > (Resend as cover letter title was missing in the first time. Sorry for noise)
> >
> > Hello,
> 
> Hi,
> 
> > This series adds Fujitsu A64FX SoC entry in drivers/soc and hardware
> > barrier driver for it.
> >
> > [Driver Description]
> >  A64FX CPU has several functions for HPC workload and hardware barrier
> >  is one of them. It is a mechanism to realize fast synchronization by
> >  PEs belonging to the same L3 cache domain by using implementation
> >  defined hardware registers.
> >  For more details, see A64FX HPC extension specification in
> >  https://github.com/fujitsu/A64FX
> >
> >  The driver mainly offers a set of ioctls to manipulate related registers.
> >  Patch 1-9 implements driver code and patch 10 finally adds kconfig,
> >  Makefile and MAINTAINER entry for the driver.
> 
> I have a number of concerns here, and at a high level, I do not think
> that this is something Linux can reasonably support in its current form.
> Sorry if this comes across as harsh; I appreciate the work that has gone
> into this, and the effort to try to upstream support is great -- my
> concerns are with the overal picture.
> 
> 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).
> 
> Secondly, the intended usage model appears to expose this to EL0 for
> direct access, and the code seems to depend on threads being pinned, but
> AFAICT this is not enforced and there is no provision for
> context-switch, thread migration, or interaction with ptrace. I fear
> this is going to be very fragile in practice, and that extending that
> support in future will require much more complexity than is currently
> apparent, with potentially invasive changes to arch code.
> 
> Thirdly, this requires userspace software to be intimately familiar with
> the HW platform that it is running on (both in terms of using IMP-DEF
> instructions and needing to know the physical layout), rather than being
> generic and portable, which I don't believe is something that we wish to
> encourage.  I also think this is unlikely to be supported by generic
> software because of the lack of portability, and consequently I struggle
> to beleive that this will see significant usage.
> 
> Further, as an IMP-DEF feature, it's not clear how much of this will
> carry forward into future designs, and where things may change. It's
> extremely difficult to determine whether any of the ABI decisions (e.g.
> the sysfs layout) are sufficient, or what level of changes would be
> necessary in userspace code if there are physical topology changes or
> changes to the strucutre of the system register interfaces.
> 
> Overall, I think this needs much more justification in terms of
> practical usage, safety/correctness, and long term maintenance, and with
> that I think the longer term goal would be to use this to justify an
> architectural feature along similar lines rather than to support any
> IMPLEMENTATION DEFINED variants upstream in Linux.
> 
> >  Also, C library and test program for this driver is available on:
> >  https://github.com/fujitsu/hardware_barrier
> 
> Hmm... I see some code in that repo which looks suspiciously like code
> from the Linux kernel tree, but licensed differently, which is
> concerning.
> 
> Specifically, the asm block in internal.h (which the SPDX headers says
> is licensed as LGPL-3.0-only) looks like a copy of code from
> arch/arm64/include/asm/sysreg.h (which is licensed as GPL-2.0-only per
> its SPDX header).
> 
> If that code was copied, I don't believe that relicensing is permitted.
> I would advise that someone with legal training considers the provenance
> of that code and what is permitted.

Sorry, I must have lacked the attention where the code comes from when I wrote the code.
I have removed that part to write assemby directly:
 https://github.com/fujitsu/hardware_barrier/blob/develop/src/internal.h
 https://github.com/fujitsu/hardware_barrier/blob/develop/src/hwblib.c#L215

Regards,
Tomohiro