[RFC] [PATCH] arm64: survive after access to unimplemented register

[Mark Rutland]

On Thu, Mar 31, 2016 at 03:28:59PM +0300, Yury Norov wrote:
> Hi Mark,
> 
> On Thu, Mar 31, 2016 at 11:05:48AM +0100, Mark Rutland wrote:
> > On Thu, Mar 31, 2016 at 05:27:03AM +0300, Yury Norov wrote:
> > > Not all vendors implement all the system registers ARM specifies.
> > 
> > The ID registers in question are precisely documented in the ARM ARM
> > (see table C5-6 in ARM DDI 0487A.i). Specifically, the ID space
> > ID_AA64MMFR2_EL1 now falls in to is listed as RAZ.
> > 
> > Any deviation from this is an erratum, and needs to be handled as such
> > (e.g. listing in silicon-errata.txt).
> > 
> > Does the issue affect ThunderX natively?
> 
> Yes, Thunder is involved, but I cannot tell more due to NDA.
> And this error is not in silicon-errata.txt.
> I'll ask permission to share more details.

Ok. Regardless of how this is solved, we need to know the details of the
erratum (and need an entry in silicon-errata.txt).

> > > So access them causes undefined instruction abort and then kernel
> > > panic. There are 3 ways to handle it we can figure out:
> > >  - use conditional compilation and erratas;
> > >  - use kernel patching;
> > >  - inline fixups resolving the abort.
> > > 
> > > Last option is more robust as it does not require additional efforts
> > > to support targers. It is looking reasonable because in many cases
> > > optional registers should be RAZ if not implemented. Special cases may
> > > be handled by underlying __read_cpuid() when needed.
> > 
> > I don't think we should do this if the only affected implementations are
> > software emulators which can be patched (and have already been, in the
> > case of QEMU).
> > 
> > In future it's very likely that early assembly code (potentially in
> > hypervisor context) will need to access ID registers which are currently
> > reserved/RAZ, and it will be rather painful to fix up accesses to this.
> 
> So we will not fix. This one fixes el1 only, and don't pretend for more. 

At some point, it's practically guaranteed that we will have to access
reserved/RAZ ID registers in other cases, so we _will_ need workarounds
that cater for those sooner or later.

We need to consider how we can handle those, in case it implies
constraints on our solution elsewhere, or requires a more complex, but
more general solution (which we can implement part of today).

For example:

* The sanity checks code will perform many back-to-back register
  accesses. Trapping lots of these could be expensive, so not performing
  the MRS at all when known to be unsafe may be preferable.

* Some registers may be read in a hot/critical path, or potentially in a
  context where we cannot handle trapping (e.g. early boot code or parts
  of KVM). In some cases, patching may be preferable to an MRS that only
  gets executed depending on a branch condition.

Before we can do any of this, we need to know the conditions of the
erratum, however.

> > Additionally, this workaround will silently mask other bugs in this area
> > (e.g. if registers like ID_AA64MMFR0_EL1 were to trap for some reason on
> > an implementation), which doesn't seem good.
> 
> We can mask it less silently, for example, print message to dmesg.
> 
> Initially I was thinking about erratas as well, but Arnd suggested
> this approach, and now think it's better. From consumer point of view,
> it's much better to have a warning line in dmesg, instead of bricked
> device, after another kernel or driver update.

Having some warning is certainly better, though I think we need to
scream _very loudly_ for cases we do not expect, as non-fatal warnings
are easily/often ignored, and can later turn out to be more critical
than previously believed.

Thanks,
Mark.