Re: [Xen-devel] [PATCH] x86/build: Use new .nops directive when available

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.08.18 at 11:55, <roger.pau@xxxxxxxxxx> wrote:
> On Wed, Aug 15, 2018 at 06:57:38PM +0100, Andrew Cooper wrote:
>> --- a/xen/arch/x86/Rules.mk
>> +++ b/xen/arch/x86/Rules.mk
>> @@ -29,6 +29,10 @@ $(call as-option-add,CFLAGS,CC,"invpcid 
>> (%rax)$$(comma)%rax",-DHAVE_AS_INVPCID)
>>  $(call as-option-add,CFLAGS,CC,\
>>      ".if ((1 > 0) < 0); .error \"\";.endif",,-DHAVE_AS_NEGATIVE_TRUE)
>>  
>> +# Check to see whether the assmbler supports the .nop directive.
>> +$(call as-option-add,CFLAGS,CC,\
>> +    ".L1: .L2: .nops (.L2 - .L1)$$(comma)9",-DHAVE_AS_NOP_DIRECTIVE)
> 
> I think I remember commenting on an earlier version of this about the
> usage of the CONTROL parameter. I would expect the assembler to
> use the most optimized version by default, is that not the case?

How could it, without knowing what the target hardware is? And even
if it could, what is considered "most optimized" tends to change every
once in a while (or else we wouldn't have different NOP flavors to
begin with).

Jan



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