Re: [PATCH] x86: Fix build following c/s 623c720f "x86: use CLFLUSHOPT when available"

[Doug Goldstein <cardoe@cardoe.com>]

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On 2/11/16 1:25 PM, Andrew Cooper wrote:
> CentOS 7 gets into trouble when compiling Xen citing:
> 
>   flushtlb.c: Assembler messages:
>   flushtlb.c:149: Error: value of 256 too large for field of 1 bytes at 1
> 
> The line number is wrong, and the error message not helpful.  It turns out
> that the intermediate generated assembly was
> 
>   # 139 "arch/x86/flushtlb.c" 1
>       661:
>       rex clflush (%r15)
>   662:
>   .pushsection .altinstructions,"a"
> 
> and it was having trouble combining the explicit REX prefix with the REX.B
> required for the use of %r15.
> 
> Follow what Linux does and use a redundant %ds prefix instead, for a final
> generated instruction of `3e 41 0f ae 3f`
> 
> While modifying this line, fix the indentation which was out by one space.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

Tested-by: Doug Goldstein <cardoe@cardoe.com>

Confirmed to fix the build issue. Results:
https://travis-ci.org/cardoe/xen/builds/108653604

> ---
> CC: Jan Beulich <JBeulich@suse.com>
> ---
>  xen/arch/x86/flushtlb.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/xen/arch/x86/flushtlb.c b/xen/arch/x86/flushtlb.c
> index 90a004f..727434e 100644
> --- a/xen/arch/x86/flushtlb.c
> +++ b/xen/arch/x86/flushtlb.c
> @@ -141,10 +141,10 @@ void flush_area_local(const void *va, unsigned int flags)
>          {
>              alternative(ASM_NOP3, "sfence", X86_FEATURE_CLFLUSHOPT);
>              for ( i = 0; i < sz; i += c->x86_clflush_size )
> -                 alternative_input("rex clflush %0",
> -                                   "data16 clflush %0",
> -                                   X86_FEATURE_CLFLUSHOPT,
> -                                   "m" (((const char *)va)[i]));
> +                alternative_input(".byte 0x3e; clflush %0", /* %ds override. */
> +                                  "data16 clflush %0",      /* clflushopt.   */
> +                                  X86_FEATURE_CLFLUSHOPT,
> +                                  "m" (((const char *)va)[i]));
>          }
>          else
>          {
> 


-- 
Doug Goldstein


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