Re: [PATCH 5/7] RISC-V: fix auipc-jalr addresses in patched alternatives

[Emil Renner Berthing <emil.renner.berthing@canonical.com>]

On Mon, 14 Nov 2022 at 12:35, Andrew Jones <ajones@ventanamicro.com> wrote:
>
> On Mon, Nov 14, 2022 at 11:57:29AM +0100, Emil Renner Berthing wrote:
> > On Thu, 10 Nov 2022 at 17:50, Heiko Stuebner <heiko@sntech.de> wrote:
> ...
> > > @@ -316,8 +384,15 @@ void __init_or_module riscv_cpufeature_patch_func(struct alt_entry *begin,
> > >                 }
> > >
> > >                 tmp = (1U << alt->errata_id);
> > > -               if (cpu_req_feature & tmp)
> > > -                       patch_text_nosync(alt->old_ptr, alt->alt_ptr, alt->alt_len);
> > > +               if (cpu_req_feature & tmp) {
> > > +                       /* do the basic patching */
> > > +                       patch_text_nosync(alt->old_ptr, alt->alt_ptr,
> > > +                                         alt->alt_len);
> > > +
> > > +                       riscv_alternative_fix_auipc_jalr(alt->old_ptr,
> > > +                                                        alt->alt_len,
> > > +                                                        alt->old_ptr - alt->alt_ptr);
> >
> > Here you're casting a void pointer to an instruction to an unsigned
> > int pointer, but since we enable compressed instructions this may
> > result in an unaligned pointer. Using this pointer will work, but may
> > be slow. Eg. fault to m-mode to be patched up. We already do that in
> > other places in the arch/riscv, but I'd prefer not to add new
> > instances of this.
>
> Alternative instruction sequences (old and new) have compression disabled.

I see, but if the instructions before the alternative sequence ends on
an unaligned address will there be inserted 16bit NOPs to make sure
the alternative sequence will be aligned?

> Thanks,
> drew

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