Re: [Patch 1/3 v2] x86/irq: local_irq_restore() should not blindly popf

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 21/10/13 19:18, Keir Fraser wrote:
> On 21/10/2013 17:33, "Andrew Cooper" <andrew.cooper3@citrix.com> wrote:
>
>> local_irq_restore() should only be concerned with possibly changing the
>> interrupt flag.  A blind popf could corrupt other system flags.
>>
>> While playing in this area, fixup an opencoded use of X86_EFLAGS_IF.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
>> CC: Keir Fraser <keir@xen.org>
>> CC: Jan Beulich <JBeulich@suse.com>
>>
>> ---
>>
>> This is rather more RFC.  It boots and runs VMs, so I am fairly sure it is
>> functionally correct, but I cant help feeling there might be a neater way to
>> do the inline assembly.  Suggestions welcome.
>> ---
>>  xen/include/asm-x86/system.h |   11 ++++++++---
>>  1 file changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/xen/include/asm-x86/system.h b/xen/include/asm-x86/system.h
>> index 6ab7d56..ff52671 100644
>> --- a/xen/include/asm-x86/system.h
>> +++ b/xen/include/asm-x86/system.h
>> @@ -3,6 +3,7 @@
>>  
>>  #include <xen/lib.h>
>>  #include <xen/bitops.h>
>> +#include <asm/processor.h>
>>  
>>  #define read_segment_register(name)                             \
>>  ({  u16 __sel;                                                  \
>> @@ -159,15 +160,19 @@ static always_inline unsigned long __cmpxchg(
>>  #define local_irq_restore(x)                                     \
>>  ({                                                               \
>>      BUILD_BUG_ON(sizeof(x) != sizeof(long));                     \
>> -    asm volatile ( "push" __OS " %0 ; popf" __OS                 \
>> -                   : : "g" (x) : "memory", "cc" );               \
>> +    asm volatile (                                               \
>> +    "pushf" __OS "\n\t"                                          \
>> +    "and" __OS " %0, (%%" __OP "sp)\n\t"                         \
>> +    "orw %1, (%%" __OP "sp)\n\t"                                 \
>> +    "popf" __OS "\n\t" : : "g" ( ~X86_EFLAGS_IF ),               \
> Would this be better as a constant constraint ("i")?

I was wondering what the best practice for this would be.

In most cases, I would imagine that an immediate would be used. 
However, as this is a define and therefore forcibly inlined everywhere
it is used, it is just possible that the compiler could find a
~X86_EFLAGS_IF already in context, and optimise down to an "and r64,r/m64".

~Andrew

>
>> +                           "g" ( x & X86_EFLAGS_IF ) );          \
>>  })
>>  
>>  static inline int local_irq_is_enabled(void)
>>  {
>>      unsigned long flags;
>>      local_save_flags(flags);
>> -    return !!(flags & (1<<9)); /* EFLAGS_IF */
>> +    return !!(flags & X86_EFLAGS_IF);
>>  }
>>  
>>  #define BROKEN_ACPI_Sx          0x0001
>