Re: riscv: hwprobe: Report Supm only when available

From: Andy Chiu <tchiu@tenstorrent.com>
To: "Radim Krcmár" <radim.krcmar@oss.qualcomm.com>
Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	Paul Walmsley <pjw@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Alexandre Ghiti <alex@ghiti.fr>,
	Samuel Holland <samuel.holland@sifive.com>,
	Andy Chiu <andybnac@gmail.com>, Guodong Xu <guodong@riscstar.com>,
	Zong Li <zong.li@sifive.com>,
	Charlie Jenkins <thecharlesjenkins@gmail.com>
Subject: Re:  riscv: hwprobe: Report Supm only when available
Date: Fri, 16 Oct 2026 16:45:00 -0500	[thread overview]
Message-ID: <atKa3JDynQmx2KS3@Tao-Chiu's-Mac> (raw)
In-Reply-To: <atKWa5PU2ECSgocI@Tao-Chiu's-Mac>

On Fri, Oct 16, 2026 at 04:26:03PM -0500, Andy Chiu wrote:
> On Tue, Oct 06, 2026 at 09:53:01AM +0200, Radim Krcmár wrote:
> > The RISC-V ISA defines Supm as
> > 
> >   "An extension that indicates that there is pointer-masking support
> >   available in user mode, with some facility provided in the application
> >   execution environment to control pointer masking."
> > 
> > Our facility always returns -EINVAL unless Supm support is configured,
> > the pointer masking ISA extension is globally present, and the user-mode
> > is running in 64-bit mode.
> > 
> > Hiding Supm in compat tasks would add complexity that doesn't seem
> > justified as 32-bit code has no reason to look at Supm anyway.
> > Reporting based on the a global sysctl toggle isn't done.
> > 
> > Do not report Supm when some harts lack the extension, because it's not
> > available on any hart in that case.
> > 
> > (Reporting Supm when only PMLEN=0 is supported isn't great, but doesn't
> >  violate the text of the extension and we ought to have Supm7 and Supm16
> >  extensions if we care about precision.)
> > 
> > Fixes: 3c2e0aff7b4f ("riscv: hwprobe: Export the Supm ISA extension")
> > Signed-off-by: Radim Krcmár <radim.krcmar@oss.qualcomm.com>
> 
> Reviewed-by: Andy Chiu <tchiu@tenstorrent.com>
> 
> > ---
> >  The compat hiding and global sysctl toggle better fit in Andy's series.
> > 
> 
> Sorry, I am not aware of any ongoing series that does the compat hiding,
> but I will keep an eye on it.

Now I know the series you mentioned. That series was dropped because the
mechanism was not in flavor of reviewers and we are going to deprecate
the prctl[1]. But I can see there is still a need for understanding if an
extension exists at runtime. We will take time to revisit that

[1]: https://lore.kernel.org/all/20261007085126.222688-1-samuel.holland@sifive.com/

Thanks,
Andy

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