Re: Re: [kvm-unit-tests PATCH v2 02/24] riscv: Initial port, hello world

[Andrew Jones <andrew.jones@linux.dev>]

On Thu, Feb 01, 2024 at 09:29:36AM +0100, Eric Auger wrote:
...
> > --- /dev/null
> > +++ b/riscv/cstart.S
> > @@ -0,0 +1,92 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * Boot entry point and assembler functions for riscv.
> > + *
> > + * Copyright (C) 2023, Ventana Micro Systems Inc., Andrew Jones <ajones@ventanamicro.com>
> > + */
> > +#include <asm/csr.h>
> > +
> > +.macro zero_range, tmp1, tmp2
> For my education what were the tmp3/4 args used for on arm?
> > +9998:	beq	\tmp1, \tmp2, 9997f
> > +	sd	zero, 0(\tmp1)
> > +	addi	\tmp1, \tmp1, 8
> > +	j	9998b
> > +9997:
> > +.endm
> > +

arm doesn't have a zero register like arm64 and riscv32/64 have, so at
least one extra tmp register is needed to hold the zero stored to the
memory. We use two tmp registers because arm has a 'strd' instruction
allowing us to write two at once, as long as the first register is
an even-numbered register and the second is the immediately following
odd-numbered register. (We should probably write a comment about the
purpose and even/odd constraints of tmp3/4 above the zero_range macro
in arm/cstart.S)

Thanks,
drew