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

[Rich Felker <dalias@libc.org>]

On Thu, Sep 16, 2021 at 04:02:50PM +0800, Vincent Chen wrote:
> On Mon, Sep 13, 2021 at 9:52 PM Rich Felker <dalias@libc.org> wrote:
> >
> > On Mon, Sep 13, 2021 at 03:44:09PM +0200, Florian Weimer via Libc-alpha wrote:
> > > * Vincent Chen:
> > >
> > > > Following the changes of struct sigcontext in Linux to reserve about 5K space
> > > > to support future ISA expansion.
> > > > ---
> > > >  sysdeps/unix/sysv/linux/riscv/sys/ucontext.h | 2 ++
> > > >  1 file changed, 2 insertions(+)
> > > >
> > > > diff --git a/sysdeps/unix/sysv/linux/riscv/sys/ucontext.h b/sysdeps/unix/sysv/linux/riscv/sys/ucontext.h
> > > > index cfafa44..80caf07 100644
> > > > --- a/sysdeps/unix/sysv/linux/riscv/sys/ucontext.h
> > > > +++ b/sysdeps/unix/sysv/linux/riscv/sys/ucontext.h
> > > > @@ -82,6 +82,8 @@ typedef struct mcontext_t
> > > >    {
> > > >      __riscv_mc_gp_state __gregs;
> > > >      union  __riscv_mc_fp_state __fpregs;
> > > > +    /* 5K + 256 reserved for vector state and future expansion.  */
> > > > +    unsigned char __reserved[5376] __attribute__ ((__aligned__ (16)));
> > > >    } mcontext_t;
> > >
> Hi Florian and Rich,
> Sorry for the late reply and thank you for reminding me the
> modification will cause ABI break.
> 
> > > 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.

VCSR is not call-saved (aka 'callee-saved' in alternate notation)
either. It's thread-local state that may be changed or left alone by
calls, and that sj/lj/ucontext functions can't touch, just like fenv.
Saving and restoring it here would be wrong.

Rich