linux-kernel - Re: [PATCH 1/2] MIPS: ebpf jit: Implement DADDI workarounds

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Message-ID: <alpine.DEB.2.21.2302271513240.63909@angie.orcam.me.uk>
Date:   Mon, 27 Feb 2023 15:17:27 +0000 (GMT)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     Johan Almbladh <johan.almbladh@...finetworks.com>
cc:     Jiaxun Yang <jiaxun.yang@...goat.com>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        linux-kernel@...r.kernel.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "paulburton@...nel.org" <paulburton@...nel.org>,
        bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH 1/2] MIPS: ebpf jit: Implement DADDI workarounds

On Mon, 27 Feb 2023, Johan Almbladh wrote:

> > > DADDI/DADDIU are only available on 64-bit CPUs, so the errata would
> > > only be applicable to that. No need for the CONFIG_64BIT conditional.
> >
> > It’s possible to compile a 32bit kernel for R4000 with CONFIG_CPU_DADDI_WORKAROUNDS
> > enabled.
> 
> Yes, but DADDI/DADDIU are 64-bit instructions so they would not be
> available when compiling the kernel in 32-bit mode for R4000, and
> hence the workaround would not be applicable, right? If this is
> correct, I would imagine CONFIG_CPU_DADDI_WORKAROUNDS itself to be
> conditional on CONFIG_64BIT. That way the this relationship is
> expressed once in the Kconfig file, instead of being spread out over
> multiple places in the code.

 It is:

	select CPU_DADDI_WORKAROUNDS if 64BIT

It only applies to 64-bit operations which are not used in 32-bit code.

  Maciej