Adding V-ext regs to signal context w/o expanding kernel struct sigcontext to avoid glibc ABI break

[Richard Henderson]

On 12/21/22 11:52, Vineet Gupta wrote:
> This is not an issue, if we don't change sigcontext (in kernel and glibc) - it is 
> essentially the case of existing binaries. kernel still saves regs on user stack, in
> rt_sigframe, its just that userspace is not able to access them in SA_SIGINFO signal
> handers. aarch64 have this implemented this approach and it is likely they can't do
> that either for SVE regs.

aarch64 can certainly access the SVE regs on the signal stack.  It simply requires that 
you parse the chain of extensions within __reserved to find it.
It's quite well designed, really.

What you can't do is "only" declare a sigcontext_t and be able to construct a new context, 
nor copy the entire context via structure assignment.

There is room within the risc-v context for a similar scheme, via

     sigcontext.sc_fpregs.q.reserved[3]

E.g.

     reserved[0] -> magic
     reserved[1] -> displacement to "extension area"
     reserved[2] -> size of "extension area"

Thus the area can be located anywhere within 4GB and expand to 4GB.
Not that I'd hope any signal frame would require 4GB.  :-)


r~