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

[Vineet Gupta]

On 12/21/22 21:32, Richard Henderson wrote:
> 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.

Yep I've been staring at it this week and really appreciate the 
extensible design. Indeed one can do thru the existing __reserved in 
sigcontext to access that from userspace.


>
> 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.  :-)

Looks like we almost missed this. Thx for the pointer Richard.

-Vineet