Re: [RFC patch 2/5] RISC-V: Reserve about 5K space in mcontext_t to support future ISA expansion.

[Florian Weimer via Libc-alpha <libc-alpha@sourceware.org>]

* Vincent Chen:

>> > This changes the size of struct ucontext_t, which is an ABI break
>> > (getcontext callers are supposed to provide their own object).
>> >
>
> The riscv vector registers are all caller-saved registers except for
> VCSR. Therefore, the struct mcontext_t needs to reserve a space for
> it. In addition, RISCV ISA is growing, so I also hope the struct
> mcontext_t has a space for future expansion. Based on the above ideas,
> I reserved a 5K space here.

You have reserved space in ucontext_t that you could use for this.

>> > This shouldn't be necessary if the additional vector registers are
>> > caller-saved.
>
> Here I am a little confused about the usage of struct mcontext_t. As
> far as I know, the struct mcontext_t is used to save the
> machine-specific information in user context operation. Therefore, in
> this case, the struct mcontext_t is allowed to reserve the space only
> for saving caller-saved registers. However, in the signal handler, the
> user seems to be allowed to use uc_mcontext whose data type is struct
> mcontext_t to access the content of the signal context. In this case,
> the struct mcontext_t may need to be the same as the struct sigcontext
> defined at kernel. However, it will have a conflict with your
> suggestion because the struct sigcontext cannot just reserve a space
> for saving caller-saved registers. Could you help me point out my
> misunderstanding? Thank you.

struct sigcontext is allocated by the kernel, so you can have pointers
in reserved fields to out-of-line start, or after struct sigcontext.

I don't know how the kernel implements this, but there is considerable
flexibility and extensibility.  The main issues comes from small stacks
which are incompatible with large register files.

Thanks,
Florian