[PATCH -next v13 10/19] riscv: Allocate user's vector context in the first-use trap

[Andy Chiu]

On Wed, Feb 15, 2023 at 3:14 PM Björn Töpel <bjorn at kernel.org> wrote:
>
> Vineet Gupta <vineetg at rivosinc.com> writes:
>
> > On 2/14/23 08:50, Björn Töpel wrote:
> >> Andy Chiu <andy.chiu at sifive.com> writes:
> >>
> >>> Hey Björn,
> >>>
> >>> On Tue, Feb 14, 2023 at 2:43 PM Björn Töpel <bjorn at kernel.org> wrote:
> >>>> So, two changes:
> >>>>
> >>>> 1. Disallow V-enablement if the existing altstack does not fit a V-sized
> >>>>     frame.
> >>> This could potentially break old programs (non-V) that load new system
> >>> libraries (with V), If the program sets a small alt stack and takes
> >>> the fault in some libraries that use V. However, existing
> >>> implementation will also kill the process when the signal arrives,
> >>> finding insufficient stack frame in such cases. I'd choose the second
> >>> one if we only have these two options, because there is a chance that
> >>> the signal handler may not even run.
> >> I think we might have different views here. A process has a pre-V, a and
> >> post-V state. Is allowing a process to enter V without the correct
> >> preconditions a good idea? Allow to run with V turned on, but not able
> >> to correctly handle a signal (the stack is too small)?
> >
> > The requirement is sane, but the issue is user experience: User trying
> > to bring up some V code has no clue that deep in some startup code some
> > alt stack had been setup and causing his process to be terminated on
> > first V code.
> >
> >>
> >> This was the same argument that the Intel folks had when enabling
> >> AMX. Sure, AMX requires *explicit* enablement, but same rules should
> >> apply, no?
> >>
> >>>> 2. Sanitize altstack changes when V is enabled.
> >>> Yes, I'd like to have this. But it may be tricky when it comes to
> >>> deciding whether V is enabled, due to the first-use trap. If V is
> >>> commonly used in system libraries then it is likely that V will be
> >>> enabled before an user set an altstack. Sanitizing this case would be
> >>> easy and straightforward.
> >
> > Good. Lets have this in v14 as it seems reasonably easy to implement.
> >
> >>> But what if the user sets an altstack before
> >>> enabling V in the first-use trap? This could happen on a statically
> >>> program that has hand-written V routines. This takes us to the 1st
> >>> question above, should we fail the user program immediately if the
> >>> altstack is set too small?
> >
> > Please lets not cross threads. We discussed this already at top. While
> > ideally required, seems tricky so lets start with post-V alt stack check.
> >
> >> For me it's obvious to fail (always) "if the altstack is too small to
> >> enable V", because it allows to execute V without proper preconditions.
> >>
> >> Personally, I prefer a stricter model. Only enter V if you can, and
> >> after entering it disallow changing the altstack.
> >>
> >> Then again, this is *my* opinion and concern. What do other people
> >> think? I don't want to stall the series.
> >
> > I concur that the alt stack checking requirements are sensible in the
> > long run. We can add the obvious check for post-V case and see if there
> > is a sane way to flag pre-V case to.
>
> Reasonable. @Andy does this resonate with you as well?
Yes, it makes sense to me. I am making this happen on v14 :)

Thanks,
Andy