linux-kernel - Re: [PATCH v2 02/14] x86/alternatives: Optimize optimize

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Date:   Sun, 21 Mar 2021 13:06:47 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, jpoimboe@...hat.com, jgross@...e.com,
        mbenes@...e.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/14] x86/alternatives: Optimize optimize_nops()

On Thu, Mar 18, 2021 at 06:11:05PM +0100, Peter Zijlstra wrote:
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -345,19 +345,39 @@ recompute_jump(struct alt_instr *a, u8 *
>  static void __init_or_module noinline optimize_nops(struct alt_instr *a, u8 *instr)
>  {
>  	unsigned long flags;
> -	int i;
> +	int nops = 0, i = 0;
> +	struct insn insn;
> +	u8 *nop = NULL;
>  
> -	for (i = 0; i < a->padlen; i++) {
> -		if (instr[i] != 0x90)
> +	do {
> +		if (insn_decode_kernel(&insn, &instr[i]))
>  			return;
> -	}
>  
> -	local_irq_save(flags);
> -	add_nops(instr + (a->instrlen - a->padlen), a->padlen);
> -	local_irq_restore(flags);
> +		if (insn.length == 1 && insn.opcode.bytes[0] == 0x90) {
> +			if (!nop) {
> +				nop = &instr[i];
> +				nops = 1;
> +			} else {
> +				nops++;
> +			}
> +		}
> +		i += insn.length;
> +
> +		if ((insn.length != 1 || i == a->instrlen) && nop) {
> +
> +			local_irq_save(flags);
> +			add_nops(nop, nops);
> +			local_irq_restore(flags);
> +
> +			DUMP_BYTES(instr, a->instrlen, "%px: [%d:%d) optimized NOPs: ",
> +				   instr, (int)(unsigned long)(nop-instr), nops);
> +
> +			nop = NULL;
> +		}
>  
> -	DUMP_BYTES(instr, a->instrlen, "%px: [%d:%d) optimized NOPs: ",
> -		   instr, a->instrlen - a->padlen, a->padlen);
> +	} while (i < a->instrlen);
> +
> +	WARN_ON_ONCE(nop);
>  }

I think I've made this simpler; pasting the whole function and not the
diff because former is easier to read:

/*
 * "noinline" to cause control flow change and thus invalidate I$ and
 * cause refetch after modification.
 *
 * Jump over the non-NOP insns, the remaining bytes must be single-byte NOPs,
 * optimize them.
 */
static void __init_or_module noinline optimize_nops(struct alt_instr *a, u8 *instr)
{
        unsigned long flags;
        struct insn insn;
        int i = 0, j;

        /* Skip preceding non-NOP instructions. */
        do {
                if (insn_decode_kernel(&insn, &instr[i]))
                        return;

                if (insn.length == 1 && insn.opcode.bytes[0] == 0x90)
                        break;

                i += insn.length;

        } while (i < a->instrlen);

        if (i >= a->instrlen - 1)
                return;

        /* Verify rest is NOPs - should not fire(tm) */
        for (j = i; j < a->instrlen - 1; j++) {
                if (WARN(instr[j] != 0x90, "Wrong insn byte 0x%hx at 0x%px\n",
                         instr[j], &instr[j]))
                        return;
        }

        local_irq_save(flags);
        add_nops(&instr[i], a->instrlen - i);
        local_irq_restore(flags);

        DUMP_BYTES(instr, a->instrlen, "%px: [%d:%d) optimized NOPs: ",
                   instr, i, a->instrlen);
}

-- 
Regards/Gruss,
    Boris.

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