Re: [PATCH v2] riscv: Add sysctl to control discard of vstate during syscall

From: Florian Weimer <fweimer@redhat.com>
To: Drew Fustini <fustini@kernel.org>
Cc: "Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Alexandre Ghiti" <alex@ghiti.fr>,
	"Samuel Holland" <samuel.holland@sifive.com>,
	"Björn Töpel" <bjorn@rivosinc.com>,
	"Andy Chiu" <andybnac@gmail.com>,
	"Conor Dooley" <conor.dooley@microchip.com>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	"Drew Fustini" <dfustini@tenstorrent.com>
Subject: Re: [PATCH v2] riscv: Add sysctl to control discard of vstate during syscall
Date: Tue, 12 Aug 2025 10:03:05 +0200	[thread overview]
Message-ID: <lhua545dl1i.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <aJpUWnYEL18dk4aC@x1> (Drew Fustini's message of "Mon, 11 Aug 2025 13:36:42 -0700")

* Drew Fustini:

>> I'm confused.  Current glibc assumes that vector registers are preserved
>> across system calls because the assembler clobbers do not mention them.
>> Similar inline assembly probably has ended up in other projects, too.
>> It works by accident if glibc is compiled for a non-vector target, or if
>> it so happens that GCC never keeps vector registers alive across system
>> calls.
>
> I wasn't trying to make any ABI changes with this sysctl patch. The
> riscv kernel documentation states vector state is not preserved across
> syscalls. I am not trying to change that policy.

Oh, then glibc is just buggy in this area.  I had no idea.  If the
vector state is already clobbered, than the sysctl for experimentation
is fine.

Thanks,
Florian

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