Re: [PATCH bpf-next v2 2/8] arm32, bpf: add support for sign-extension load instruction

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Wed, Sep 06, 2023 at 06:33:14PM +0000, Puranjay Mohan wrote:
> The cpuv4 added the support of an instruction that is similar to load
> but also sign-extends the result after the load.
> 
> BPF_MEMSX | <size> | BPF_LDX means dst = *(signed size *) (src + offset)
> here <size> can be one of BPF_B, BPF_H, BPF_W.
> 
> ARM32 has instructions to load a byte or a half word with sign
> extension into a 32bit register. As the JIT uses two 32 bit registers
> to simulate a 64-bit BPF register, an extra instruction is emitted to
> sign-extent the result up to the second register.
> 
> Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

but see below...

Thanks!

> +static inline void emit_ldsx_r(const s8 dst[], const s8 src,
> +			       s16 off, struct jit_ctx *ctx, const u8 sz){
> +	const s8 *tmp = bpf2a32[TMP_REG_1];
> +	const s8 *rd = is_stacked(dst_lo) ? tmp : dst;
> +	s8 rm = src;
> +	int add_off;
> +
> +	if (!is_ldst_imm8(off, sz)) {

I think a comment here would be useful:
		/* offset does not fit in the load/store immediate,
		 * construct an ADD instruction to apply the offset.
		 */

otherwise I'm sure someone will question why we aren't handling the zero
case below... since zero will fit in the load/store immediate.

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