[RISC-V] [tech-j-ext] [RFC PATCH 5/9] riscv: Split per-CPU and per-thread envcfg bits

[Deepak Gupta]

> > >
> > > And instead of context switching in `_switch_to`,
> > > In `entry.S` pick up `envcfg` from `thread_info` and write it into CSR.
> >
> > The immediate reason is that writing envcfg in ret_from_exception() adds cycles
> > to every IRQ and system call exit, even though most of them will not change the
> > envcfg value. This is especially the case when returning from an IRQ/exception
> > back to S-mode, since envcfg has zero effect there.
> >
> > The CSRs that are read/written in entry.S are generally those where the value
> > can be updated by hardware, as part of taking an exception. But envcfg never
> > changes on its own. The kernel knows exactly when its value will change, and
> > those places are:
> >
> >  1) Task switch, i.e. switch_to()
> >  2) execve(), i.e. start_thread() or flush_thread()
> >  3) A system call that specifically affects a feature controlled by envcfg
>
> Yeah I was optimizing for a single place to write instead of
> sprinkling at multiple places.
> But I see your argument. That's fine.
>

Because this is RFC and we are discussing it. I thought a little bit
more about this.

If we were to go with the above approach that essentially requires
whenever a envcfg bit changes, `sync_envcfg`
has to be called to reflect the correct value.

What if some of these features enable/disable are exposed to `ptrace`
(gdb, etc use cases) for enable/disable.
How will syncing work then ?

I can see the reasoning behind saving some cycles during trap return.
But `senvcfg` is not actually a user state, it
controls the execution environment configuration for user mode. I
think the best place for this CSR to be written is
trap return and writing at a single place from a single image on stack
reduces chances of bugs and errors. And allows
`senvcfg` features to be exposed to other kernel flows (like `ptrace`)

We can figure out ways on how to optimize in trap return path to avoid
writing it if we entered and exiting on the same
task.