Re: [Xen-devel] [PATCH] x86/setup: Correct register clobbers for the asm statement when resyncing the stack

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 21/10/14 09:44, Jan Beulich wrote:
>>>> On 20.10.14 at 19:30, <andrew.cooper3@xxxxxxxxxx> wrote:
>> When resyncing the stack, the asm statement does not identify %rsi, %rdi and
>> %rcx as clobbered by the 'rep movsq'.
>>
>> Luckily, there are no functional problems in the generated code.  GCC 
>> decides
>> not to save any of them before calling boostrap_map(), which clobbers them.
>>
>> Correct the clobbers, by listing them as earlyclobber discarded outputs.
>>
>> Reported-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> CC: Keir Fraser <keir@xxxxxxx>
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>
>> ---
>>
>> I have done an audit of the other uses of `rep $STRINGOP` and this is the 
>> only
>> asm statement with incorrect clobbers.
>> ---
>>  xen/arch/x86/setup.c |    9 ++++++---
>>  1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
>> index 8c8b91f..f470c4a 100644
>> --- a/xen/arch/x86/setup.c
>> +++ b/xen/arch/x86/setup.c
>> @@ -836,6 +836,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
>>              l2_pgentry_t *pl2e;
>>              uint64_t load_start;
>>              int i, j, k;
>> +            long _discard;
> I can't see why you couldn't use i, j, or k for the discarding purposes.

A parameter with the name "_discard" is far more explicit about its
purpose when read as part of the output parameter list.

I could switch to i and leave a comment by the parameters if you insist.

>
>> @@ -902,11 +903,13 @@ void __init noreturn __start_xen(unsigned long mbi_p)
>>                  "movq %%cr4,%%rsi ; "
>>                  "andb $0x7f,%%sil ; "
>>                  "movq %%rsi,%%cr4 ; " /* CR4.PGE == 0 */
>> -                "movq %0,%%cr3    ; " /* CR3 == new pagetables */
>> +                "movq %6,%%cr3    ; " /* CR3 == new pagetables */
>>                  "orb $0x80,%%sil  ; "
>>                  "movq %%rsi,%%cr4   " /* CR4.PGE == 1 */
>> -                : : "r" (__pa(idle_pg_table)), "S" (cpu0_stack),
>> -                "D" (__va(__pa(cpu0_stack))), "c" (STACK_SIZE / 8) : 
>> "memory" );
>> +                : "=&S"(_discard), "=&D"(_discard), "=&c"(_discard)
>> +                : "0"(cpu0_stack), "1"(__va(__pa(cpu0_stack))),
>> +                  "2"(STACK_SIZE / 8), "r"(__pa(idle_pg_table))
>> +                : "memory" );
> Among the inputs, please put the one actively used by number first,
> or convert to labeled operands.

As far as I can tell, it is impossible to get the idle pagetables to
parameter %0 as it is not present in the output list.  I have switched
to a named parameter.

>  Also please don't corrupt the
> formatting - there ought to be a blank between the closing constraint
> quote and the opening parenthesis in all operands.

Done

~Andrew


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