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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 11/02/16 19:25, Andrew Cooper 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.
>
> 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>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> ---
>  xen/arch/x86/flushtlb.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/xen/arch/x86/flushtlb.c b/xen/arch/x86/flushtlb.c
> index 90a004f..727434e 100644
> --- 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",
                 ^
/local/xen.git/xen/include/asm/alternative.h:98:16: note: expanded from
macro 'alternative_input'
        asm volatile (ALTERNATIVE(oldinstr, newinstr, feature)          \
                      ^
/local/xen.git/xen/include/asm/alternative.h:60:2: note: expanded from
macro 'ALTERNATIVE'
        OLDINSTR(oldinstr)                                                \
        ^
/local/xen.git/xen/include/asm/alternative.h:28:40: note: expanded from
macro 'OLDINSTR'
#define OLDINSTR(oldinstr)      "661:\n\t" oldinstr "\n662:\n"
                                       ^
<inline asm>:2:2: note: instantiated into assembly here
        rex clflush (%r15,%rdx)
        ^~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

~Andrew

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