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

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Message-ID: <lhua545dl1i.fsf@oldenburg.str.redhat.com>
Date: Tue, 12 Aug 2025 10:03:05 +0200
From: Florian Weimer <fweimer@...hat.com>
To: Drew Fustini <fustini@...nel.org>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,  Palmer Dabbelt
 <palmer@...belt.com>,  Alexandre Ghiti <alex@...ti.fr>,  Samuel Holland
 <samuel.holland@...ive.com>,  Björn Töpel
 <bjorn@...osinc.com>,  Andy Chiu
 <andybnac@...il.com>,  Conor Dooley <conor.dooley@...rochip.com>,
  linux-riscv@...ts.infradead.org,  linux-kernel@...r.kernel.org,  Drew
 Fustini <dfustini@...storrent.com>
Subject: Re: [PATCH v2] riscv: Add sysctl to control discard of vstate
 during syscall

* 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