Auto-enabling V unit and/or use of elf attributes (was Re: Adding V-ext regs to signal context w/o expanding kernel struct sigcontext to avoid glibc ABI break)

[Andy Chiu]

On Wed, Jan 11, 2023 at 1:07 PM Jeff Law <jlaw at ventanamicro.com> wrote:
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>
>
> On 1/10/23 21:57, Richard Henderson wrote:
> > On 1/10/23 20:28, Jeff Law wrote:
> >>
> >>
> >> On 1/10/23 18:22, Richard Henderson wrote:
> >>> On 1/10/23 10:07, Vineet Gupta wrote:
> >>>> Yes bulk of glibc might not have vector code, but those V ifunc
> >>>> routines do and IMO this information needs to be recorded somewhere
> >>>> in the elf. Case in point being the current issue with how to enable
> >>>> V unit. Community wants a per-process enable, using an explicit
> >>>> prctl from userspace (since RV doesn't have fault-on-first use
> >>>> hardware mechanism unlike some of the other arches). But how does
> >>>> the glibc loader know to invoke prctl. We can't just rely on user
> >>>> env GLIBC_TUNABLE etc since that might not be accurate. It needs
> >>>> somethign concrete which IMO can come from elf attributes. If not,
> >>>> do you have suggestions on how to solve this issue ?
> >>>
> >>> Why not just fault on first use to enable?  That's vastly less
> >>> complicated than trying to plumb anything through elf resulting in a
> >>> prctl.
> >> Well, the answer is in Vineet's paragraph -- the hardware apparently
> >> doesn't have fault-on-first-use which is mighty unfortunate.
> >
> > Nonsense -- sstatus.vs stores {off, initial, clean, dirty} state, just
> > like fpu.
> > Now treat the vector unit just like fpu lazy migration.
> Then let's do something sensible.    Manually enabling via prctl seems
> silly if we have fault on first use.
Yes, faulting on first use is a viable way of approaching. However, my
concern is that doing this on a system with libraries having common
V-optimized routines such as memcpy, memset would essentially trap
every process to m-mode starting up. This might take more cost than a
prctl syscall. And if every process on the system wants to be
benefited from V-optimized ifuncs, then having an additional prctl to
call at start time seems tedious as well.

Andy