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

From: "Heiko Stübner" <heiko@sntech.de>
To: Emil Renner Berthing <emil.renner.berthing@canonical.com>,
	Andrew Jones <ajones@ventanamicro.com>
Cc: linux-riscv@lists.infradead.org, palmer@dabbelt.com,
	christoph.muellner@vrull.eu, prabhakar.csengg@gmail.com,
	conor@kernel.org, philipp.tomsich@vrull.eu
Subject: Re: [PATCH 5/7] RISC-V: fix auipc-jalr addresses in patched alternatives
Date: Mon, 14 Nov 2022 12:38:39 +0100	[thread overview]
Message-ID: <2234530.yKVeVyVuyW@diego> (raw)
In-Reply-To: <20221114113553.3dxrumhk72jouzek@kamzik>

Am Montag, 14. November 2022, 12:35:53 CET schrieb Andrew Jones:
> 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.

That was my first thought as well, but I think Emil was talking more about the
placement of the alternative block inside the running kernel.

i.e. I guess the starting point of an alternative sequence could also be unaligned.

Though I don't _yet_ see how an improvement could look like.

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