linux-kernel - Re: [PATCH RFC] arm64: improve display about CPU architecture in cpuinfo

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Date:   Mon, 07 Mar 2022 12:23:20 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Rongwei Wang <rongwei.wang@...ux.alibaba.com>
Cc:     catalin.marinas@....com, will@...nel.org, joey.gouly@....com,
        mark.rutland@....com, akpm@...ux-foundation.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] arm64: improve display about CPU architecture in cpuinfo

On Mon, 07 Mar 2022 12:13:50 +0000,
Rongwei Wang <rongwei.wang@...ux.alibaba.com> wrote:
> 
> 
> 
> On 3/7/22 4:45 PM, Marc Zyngier wrote:
> > On Mon, 07 Mar 2022 03:04:17 +0000,
> > Rongwei Wang <rongwei.wang@...ux.alibaba.com> wrote:
> >> 
> >> Now, it is unsuitable for both ARMv8 and ARMv9 to show a
> >> fixed string "CPU architecture: 8" in /proc/cpuinfo.
> > 
> > Please read the various threads that have been going on over the past
> > 10+ years about *why* we don't allow this sort of change (TL;DR: it
> > breaks userspace, and we don't do that).
> > 
> > Also, there is no material difference between v8 and v9 that would be
> > observable from userspace outside of the "Features:" line. And if that
> > doesn't convince you, just think of '8' as the number of bytes
> > used by

> I got your point. It seems that we can regard '8' as the number of
> bytes. But what make me do this is that 'CPU architecture: 8' is
> confusing, especially those responsible for testing.
> And I believe that most people regard this '8' as ARMv8, maybe not.

That was the original intention. But given that there is no userspace
visible difference between v8, v9 and  whatever comes after it, this
is a pointless change. My comment about the size of a VA was just a
joke, and not something to be taken seriously.

> In fact, I'm not sure it has potential ABI implications, so seek your
> advice.

Plenty of userspace programs parse /proc/cpuinfo. If you replace '8'
with anything else, they will fail. Which is why although your patch
makes sense, it comes 10 years too late, and we can't change this
anymore.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.