[PATCH v2 1/1] riscv: Add RVV memset via multiarch/IFUNC

[Darius Rad]

On Tue, Nov 04, 2025 at 04:44:59PM +0100, Adhemerval Zanella Netto wrote:
> 
> 
> On 04/11/25 11:33, Darius Rad wrote:
> > On Tue, Nov 04, 2025 at 08:37:49PM +0800, Yao Zihong wrote:
> >> This patch adds an RVV-optimized implementation of memset for RISC-V and
> >> enables it through the existing multiarch/IFUNC mechanism.
> >>
> >> The implementation integrates Hau Hsu’s 2023 RVV work under a unified
> >> ifunc-based framework. A vectorized version (__memset_vector) is added
> >> alongside the generic fallback (__memset_generic). The runtime resolver
> >> selects the RVV variant when RISCV_HWPROBE_KEY_IMA_EXT_0 reports vector
> >> support (V).
> > 
> > This needs to use dl_hwcap, as I said before [1].
> > 
> > [1] https://sourceware.org/pipermail/libc-alpha/2025-October/171687.html
> > 
> >>
> >> Tested on MUSE-Pi (SpacemiT M1) and verified by Jeff Law on BPI and
> >> Pioneer boards with no regressions compared with glibc master.
> > 
> > This patch certainly has regressions compared with glibc master.
> > Presently, if one has an environment that implements the vector extension,
> > but it is disabled at runtime with prctl, glibc will operate properly.
> > After this patch, running in such an environment will trigger an illegal
> > instruction fault whenever memset is run.
> > 
> 
> How do you propose to fix PR_RISCV_V_VSTATE_CTRL_OFF for function 
> multiversioning [1]? Last time I checked libgcc only uses risc_hwprobes [2] 
> to do the function selection.

It could use getauxval(AT_HWCAP), as the documentation states [1]:

To get the availability of V in an ELF program, please read
:c:macro:`COMPAT_HWCAP_ISA_V` bit of :c:macro:`ELF_HWCAP` in the auxiliary
vector.

> 
> There is also all __riscv_hwprobe consumers that might only assume it as the
> authoritative source of hardware support.
> 

That would be their fault for making a wrong assumption and their bug to
fix.  The prctl enable for vector has been a part of Linux for as long as
RISCV_HWPROBE_IMA_V has.


[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/arch/riscv/vector.rst