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)

[Jeff Law]

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.

Jeff