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

[Vineet Gupta]

On 11/5/25 11:36, Jeff Law wrote:
> On 11/5/25 12:23 PM, Darius Rad wrote:
>
>>> We've had approval for the riscv probe stuff for roughly a year, the only
>>> reason it hasn't moved forward is I didn't have the time to push the
>>> approved patch into the tree.  Yao has kindly picked up the effort and it's
>>> ready to move forward.   hwprobe is the approved mechanism to handle this
>>> stuff across the various relevant projects.  If there's capabilities missing
>>> in that interface, then that's what needs to be fixed.  But we shouldn't
>>> make perfect be the enemy of the good IMHO.
>> So do that first, or use dl_hwcap until it is available.  Don't break
>> something that's working.
> But that is precisely the code you are insisting Yao change.  That code 
> uses hwprobe in both paths and totally avoids dl_hwcap per the agreed 
> upon design.
>
> User space turning the vector unit on/off, while interesting, should not 
> drive this discussion.  That needs to go back to the kernel and glibc 
> teams to figure out if/how it should be supported within the hwprobe 
> framework.

Perhaps I'm wrong, but I feel obliged to interject my 2 cents :  Do we have a
real issue here ?

Quoting Darius' original concern

" 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."

Vector unit being turned off at the beginning is just an implementation detail
to lazily enable it (using fault on first use).
So prctl's initial disable/enable state doesn't matter IMO. The ifunc selector
would use hwprobe and see if kernel is advertising V as available. If it
available, but disabled by prctl, it should still go ahead with wiring up the V
variants - first use and fault will just work.

Am I missing something here ?

Thx,
-Vineet