linux-kernel - Re: [PATCH 4/9] x86/alternative: Implement .retpoline

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Date:   Fri, 15 Oct 2021 16:24:08 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, jpoimboe@...hat.com, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, alexei.starovoitov@...il.com,
        ndesaulniers@...gle.com
Subject: Re: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support

On Wed, Oct 13, 2021 at 02:22:21PM +0200, Peter Zijlstra wrote:
> +static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
> +{
> +	void (*target)(void);
> +	int reg, i = 0;
> +
> +	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
> +		return -1;
> +
> +	target = addr + insn->length + insn->immediate.value;
> +	reg = (target - &__x86_indirect_thunk_rax) /
> +	      (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax);

I guess you should compute those values once so that it doesn't have to
do them for each function invocation. And it does them here when I look
at the asm it generates.

> +
> +	if (WARN_ON_ONCE(reg & ~0xf))
> +		return -1;

Sanity-checking the alignment of those thunks?

> +
> +	i = emit_indirect(insn->opcode.bytes[0], reg, bytes);
> +	if (i < 0)
> +		return i;
> +
> +	for (; i < insn->length;)
> +		bytes[i++] = BYTES_NOP1;

Why not:

        nop_len = insn->length - i;
        if (nop_len) {
                memcpy(&bytes[i], x86_nops[nop_len], nop_len);
                i += nop_len;
        }

and then you save yourself the optimize_nops() call because it'll take
the right-sized NOP directly.

-- 
Regards/Gruss,
    Boris.

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