linux-kernel - Re: [PATCH v5] RISC-V: Show accurate per-hart isa in /proc/cpuinfo

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Message-ID: <20230826-anguished-tutu-81d63b3081a7@spud>
Date:   Sat, 26 Aug 2023 00:26:25 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Evan Green <evan@...osinc.com>
Cc:     Palmer Dabbelt <palmer@...osinc.com>,
        Anup Patel <apatel@...tanamicro.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Heiko Stuebner <heiko@...ech.de>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Conor Dooley <conor.dooley@...rochip.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        linux-riscv@...ts.infradead.org,
        Andrew Jones <ajones@...tanamicro.com>
Subject: Re: [PATCH v5] RISC-V: Show accurate per-hart isa in /proc/cpuinfo

On Fri, Aug 25, 2023 at 04:11:38PM -0700, Evan Green wrote:
> In /proc/cpuinfo, most of the information we show for each processor is
> specific to that hart: marchid, mvendorid, mimpid, processor, hart,
> compatible, and the mmu size. But the ISA string gets filtered through a
> lowest common denominator mask, so that if one CPU is missing an ISA
> extension, no CPUs will show it.
> 
> Now that we track the ISA extensions for each hart, let's report ISA
> extension info accurately per-hart in /proc/cpuinfo. We cannot change
> the "isa:" line, as usermode may be relying on that line to show only
> the common set of extensions supported across all harts. Add a new "hart
> isa" line instead, which reports the true set of extensions for that
> hart.
> 
> Signed-off-by: Evan Green <evan@...osinc.com>
> Reviewed-by: Andrew Jones <ajones@...tanamicro.com>

> Reviewed-by: Conor Dooley <conor.dooley@...rochip.com>

Can you drop this if you repost?

> +"isa" vs "hart isa" lines in /proc/cpuinfo
> +------------------------------------------
> +
> +The "isa" line in /proc/cpuinfo describes the lowest common denominator of
> +RISC-V ISA extensions recognized by the kernel and implemented on all harts. The
> +"hart isa" line, in contrast, describes the set of extensions recognized by the
> +kernel on the particular hart being described, even if those extensions may not
> +be present on all harts in the system.

> In both cases, the presence of a feature
> +in these lines guarantees only that the hardware has the described capability.
> +Additional kernel support or policy control changes may be required before a
> +feature is fully usable by userspace programs.

I do not think that "in both cases" matches the expectations of
userspace for the existing line. It's too late at night for me to think
properly, but I think our existing implementation does work like you
have documented for FD/V. I think I previously mentioned that it could
misreport things for vector during the review of the vector series but
forgot about it until now.

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