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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 12/02/16 08:13, Jan Beulich wrote:
>>>> 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).

That is tangential to the point.  The useful part is the end message:

> <inline asm>:2:2: note: instantiated into assembly here
>         rex clflush (%r15,%rdx)
>         ^~~~~~~~~~~~~~~~~~~~~~~

which makes it substantially more clear that there is both a REX prefix
and REX.B required.

~Andrew

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