Re: [Xen-devel] [PATCH 2/7] x86/alt: Clean up struct alt_instr and its users

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Feb 12, 2018 at 11:23:02AM +0000, Andrew Cooper wrote:
>  * Rename some fields for consistency and clarity, and use standard types.
>  * Don't opencode the use of ALT_{ORIG,REPL}_PTR().
> 
> No functional change.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> ---
>  xen/arch/x86/alternative.c        | 24 ++++++++++++------------
>  xen/include/asm-x86/alternative.h | 12 ++++++------
>  2 files changed, 18 insertions(+), 18 deletions(-)
> 
> diff --git a/xen/arch/x86/alternative.c b/xen/arch/x86/alternative.c
> index 5c8b6f6..f8ddab5 100644
> --- a/xen/arch/x86/alternative.c
> +++ b/xen/arch/x86/alternative.c
> @@ -163,8 +163,6 @@ void init_or_livepatch apply_alternatives(const struct 
> alt_instr *start,
>                                            const struct alt_instr *end)
>  {
>      const struct alt_instr *a;
> -    u8 *instr, *replacement;
> -    u8 insnbuf[MAX_PATCH_LEN];
>  
>      printk(KERN_INFO "alt table %p -> %p\n", start, end);
>  
> @@ -179,23 +177,25 @@ void init_or_livepatch apply_alternatives(const struct 
> alt_instr *start,
>       */
>      for ( a = start; a < end; a++ )
>      {
> -        instr = (u8 *)&a->instr_offset + a->instr_offset;
> -        replacement = (u8 *)&a->repl_offset + a->repl_offset;
> -        BUG_ON(a->replacementlen > a->instrlen);
> -        BUG_ON(a->instrlen > sizeof(insnbuf));
> +        uint8_t *orig = ALT_ORIG_PTR(a);
> +        uint8_t *repl = ALT_REPL_PTR(a);
> +        uint8_t buf[MAX_PATCH_LEN];
> +
> +        BUG_ON(a->repl_len > a->orig_len);
> +        BUG_ON(a->orig_len > sizeof(buf));

Would be nice to have an assembly BUILD_BUG_ON equivalent so that this
could be turned into a compile time check and added to ALTINSTR_ENTRY.

>          BUG_ON(a->cpuid >= NCAPINTS * 32);
> +
>          if ( !boot_cpu_has(a->cpuid) )
>              continue;
>  
> -        memcpy(insnbuf, replacement, a->replacementlen);
> +        memcpy(buf, repl, a->repl_len);
>  
>          /* 0xe8/0xe9 are relative branches; fix the offset. */
> -        if ( a->replacementlen >= 5 && (*insnbuf & 0xfe) == 0xe8 )
> -            *(s32 *)(insnbuf + 1) += replacement - instr;
> +        if ( a->repl_len >= 5 && (*buf & 0xfe) == 0xe8 )
> +            *(s32 *)(buf + 1) += repl - orig;

(int32_t *)

>  
> -        add_nops(insnbuf + a->replacementlen,
> -                 a->instrlen - a->replacementlen);
> -        text_poke(instr, insnbuf, a->instrlen);
> +        add_nops(buf + a->repl_len, a->orig_len - a->repl_len);
> +        text_poke(orig, buf, a->orig_len);
>      }
>  }
>  
> diff --git a/xen/include/asm-x86/alternative.h 
> b/xen/include/asm-x86/alternative.h
> index 13ac104..fefa87d 100644
> --- a/xen/include/asm-x86/alternative.h
> +++ b/xen/include/asm-x86/alternative.h
> @@ -9,15 +9,15 @@
>  #include <xen/types.h>
>  
>  struct alt_instr {
> -    s32 instr_offset;       /* original instruction */
> -    s32 repl_offset;        /* offset to replacement instruction */
> -    u16 cpuid;              /* cpuid bit set for replacement */
> -    u8  instrlen;           /* length of original instruction */
> -    u8  replacementlen;     /* length of new instruction, <= instrlen */
> +    int32_t  orig_offset;   /* original instruction */
> +    int32_t  repl_offset;   /* offset to replacement instruction */
> +    uint16_t cpuid;         /* cpuid bit set for replacement */
> +    uint8_t  orig_len;      /* length of original instruction */
> +    uint8_t  repl_len;      /* length of new instruction, <= instrlen */
>  };
>  
>  #define __ALT_PTR(a,f)      ((u8 *)((void *)&(a)->f + (a)->f))

While there maybe you could also change the above to uint8_t...

> -#define ALT_ORIG_PTR(a)     __ALT_PTR(a, instr_offset)
> +#define ALT_ORIG_PTR(a)     __ALT_PTR(a, orig_offset)
>  #define ALT_REPL_PTR(a)     __ALT_PTR(a, repl_offset)
>  
>  extern void add_nops(void *insns, unsigned int len);
> -- 
> 2.1.4
> 

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