[PATCH v2] ARM: entry: omit FP emulation for UND exceptions taken in kernel mode

[Nick Desaulniers]

On Wed, Nov 18, 2020 at 9:00 AM Ard Biesheuvel <ardb at kernel.org> wrote:
>
> On Wed, 18 Nov 2020 at 17:59, Nick Desaulniers <ndesaulniers at google.com> wrote:
> >
> > On Wed, Nov 18, 2020 at 8:48 AM Ard Biesheuvel <ardb at kernel.org> wrote:
> > >
> > > On Wed, 18 Nov 2020 at 17:42, Nick Desaulniers <ndesaulniers at google.com> wrote:
> > > >
> > > > On Wed, Nov 18, 2020 at 5:09 AM Ard Biesheuvel <ardb at kernel.org> wrote:
> > > > >
> > > > > There are a couple of problems with the exception entry code that deals
> > > > > with FP exceptions (which are reported as UND exceptions) when building
> > > > > the kernel in Thumb2 mode:
> > > > > - the conditional branch to vfp_kmode_exception in vfp_support_entry()
> > > > >   may be out of range for its target, depending on how the linker decides
> > > > >   to arrange the sections;
> > > > > - when the UND exception is taken in kernel mode, the emulation handling
> > > > >   logic is entered via the 'call_fpe' label, which means we end up using
> > > > >   the wrong value/mask pairs to match and detect the NEON opcodes.
> > > > >
> > > > > Since UND exceptions in kernel mode are unlikely to occur on a hot path
> > > > > (as opposed to the user mode version which is invoked for VFP support
> > > > > code and lazy restore), we can use the existing undef hook machinery for
> > > >
> > > > Right, I'd expect these maybe from userspace, but within the kernel?
> > > >
> > >
> > > Russell explained off-list that there used to be a case in the pre-VFP
> > > era, but this is no longer relevant.
> >
> > If the use case is no longer relevant, consider dropping support.
> > Dead code is technical debt.
> >
>
> Dropping support for what?

By `there used to be a case`, I interpret "a case" to mean one case,
singular.  "but this is no longer relevant" seems to imply that
singular case is not an issue anymore.

If what you're referring to is "UND exceptions in kernel mode," then I
guess we need exception handling support for those, even if they occur
less frequently than the "pre-VFP era" alluded to.  So nvm
-- 
Thanks,
~Nick Desaulniers