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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.02.16 at 20:41, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 11/02/16 19:25, Andrew Cooper wrote:
>> --- 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
>>          {
> 
> It turns out that Clang is far more useful at diagnosing this issue than
> GCC.
> 
> flushtlb.c:144:18: error: invalid instruction mnemonic 'rex'
>                  alternative_input("rex clflush %0",
>                  ^

Except that 'rex' is by no means invalid. If anything Clang's internal
assembler doesn't support it (and hence is not gas compatible).

Jan


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