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 08:23, 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" ?

>  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.

>
>> 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@xxxxxxxxxx>
> I think before committing I'll extend your patch by introducing
> and using Linux'es NOP_DS_PREFIX.

Ok.

~Andrew

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