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)

[Richard Henderson]

On 1/10/23 17:22, Richard Henderson wrote:
> And, surely, there's some minimal vector configuration for which the allocation must succeed.

To answer my own question here, no, there does not seem to be a way to cap VLMAX in the OS 
or the hypervisor -- vsetvli rd, r0, e8 will always set VL to the VLMAX for which the cpu 
is configured.

(ARM SVE can artificially limit the vector length.  Linux chooses the default vector 
length so that state fits within the existing 4k signal stack frame.  This is good enough 
for the vector usage within e.g. strlen.  In order to take advantage of any larger vector 
length the hardware may support, one must use a prctl.)


r~