linux-kernel - Re: [PATCH 1/2] x86: Remove dynamic NOP selection

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Date:   Fri, 12 Mar 2021 13:09:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, rostedt@...dmis.org, hpa@...or.com,
        torvalds@...uxfoundation.org
Cc:     linux-kernel@...r.kernel.org, linux-toolchains@...r.kernel.org,
        jpoimboe@...hat.com, alexei.starovoitov@...il.com,
        mhiramat@...nel.org
Subject: Re: [PATCH 1/2] x86: Remove dynamic NOP selection

On Fri, Mar 12, 2021 at 12:32:54PM +0100, Peter Zijlstra wrote:
> +#ifndef CONFIG_64BIT
> +
> +/*
> + * Generic 32bit nops from GAS:
> + *
> + * 1: nop
> + * 2: movl %esi,%esi
> + * 3: leal 0x0(%esi),%esi
> + * 4: leal 0x0(%esi,%eiz,1),%esi
> + * 5: leal %ds:0x0(%esi,%eiz,1),%esi
> + * 6: leal 0x0(%esi),%esi
> + * 7: leal 0x0(%esi,%eiz,1),%esi
> + * 8: leal %ds:0x0(%esi,%eiz,1),%esi
>   *
> + * Except 5 and 8, which are DS prefixed 4 and 7 resp, where GAS would emit 2
> + * nop instructions.
>   */
> +#define BYTES_NOP1	0x90
> +#define BYTES_NOP2	0x89,0xf6
> +#define BYTES_NOP3	0x8d,0x76,0x00
> +#define BYTES_NOP4	0x8d,0x74,0x26,0x00
> +#define BYTES_NOP5	0x3e,BYTES_NOP4
> +#define BYTES_NOP6	0x8d,0xb6,0x00,0x00,0x00,0x00
> +#define BYTES_NOP7	0x8d,0xb4,0x26,0x00,0x00,0x00,0x00
> +#define BYTES_NOP8	0x3e,BYTES_NOP7
> +
> +#else
>  
> +/*
> + * Generic 64bit nops from GAS:
> + *
> + * 1: nop
> + * 2: osp nop
> + * 3: nopl (%eax)
> + * 4: nopl 0x00(%eax)
> + * 5: nopl 0x00(%eax,%eax,1)
> + * 6: osp nopl 0x00(%eax,%eax,1)
> + * 7: nopl 0x00000000(%eax)
> + * 8: nopl 0x00000000(%eax,%eax,1)
> + */
> +#define BYTES_NOP1	0x90
> +#define BYTES_NOP2	0x66,BYTES_NOP1
> +#define BYTES_NOP3	0x0f,0x1f,0x00
> +#define BYTES_NOP4	0x0f,0x1f,0x40,0x00
> +#define BYTES_NOP5	0x0f,0x1f,0x44,0x00,0x00
> +#define BYTES_NOP6	0x66,BYTES_NOP5
> +#define BYTES_NOP7	0x0f,0x1f,0x80,0x00,0x00,0x00,0x00
> +#define BYTES_NOP8	0x0f,0x1f,0x84,0x00,0x00,0x00,0x00,0x00
>  
> +#endif /* CONFIG_64BIT */

Note that this also made all NOPs single instructions and removed the
special atomic nop.