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

[Conor Dooley]

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@xxxxxxxxxxxx>
> Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>

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

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.

Attachment: signature.asc
Description: PGP signature