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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.02.16 at 10:51, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 12/02/16 08:23, Jan Beulich wrote:
>>>>> On 11.02.16 at 20:25, <andrew.cooper3@xxxxxxxxxx> 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.
>> What gas version is this? I just checked with 2.20, which has no
>> problem combining an explicit with a generated REX prefix.
> 
> bash-4.2$ as --version
> GNU assembler version 2.23.52.0.1-30.el7_1.2 20130226
> 
> 
>>  Or
>> wait, no, your description of the issue is wrong: It actually is the
>> folding of the two REX prefixes which causes the problem,
> 
> This is what I said.  What do you think I meant with "trouble combining
> the" ?

Argh - I meant to say "It actually isn't ...".

>>  since
>> that results in the replacement instruction being one byte longer
>> than the to be replaced one.
> 
> But that is still the case with an explicit %ds override.  The assembler
> still has to insert an extra byte somehow.

No. We now always have one non-REX prefix, and both instructions
will have the same REX/ModRM/SIB encoding.

Jan


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