Re: [PATCH v1 07/14] xen/riscv: introduce exception handlers implementation

[Oleksii]

On Mon, 2023-01-23 at 11:50 +0000, Andrew Cooper wrote:
> 
> 
> > +        /* Save context to stack */
> > +        REG_S   sp, (RISCV_CPU_USER_REGS_OFFSET(sp) -
> > RISCV_CPU_USER_REGS_SIZE) (sp)
> > +        addi    sp, sp, -RISCV_CPU_USER_REGS_SIZE
> > +        REG_S   t0, RISCV_CPU_USER_REGS_OFFSET(t0)(sp)
> 
> Exceptions on RISC-V don't adjust the stack pointer.  This logic
> depends
> on interrupting Xen code, and Xen not having suffered a stack
> overflow
> (and actually, that the space on the stack for all registers also
> doesn't overflow).
> 
Probably I missed something but an idea of the code above was to
reserve memory on a stack to save the registers which can be changed
in __handler_expception() as the line of code where exception occurs
will expect that registers value weren't changed.
Otherwise if we won't reserve memory on stack it will be corrupted by
REG_S which basically is SD instruction.