Re: [Xen-devel] [PATCH 1/3] x86/irq: local_irq_restore() should not blindly popf

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 21/10/2013 14:58, "David Vrabel" <david.vrabel@xxxxxxxxxx> wrote:

> On 21/10/13 14:41, Andrew Cooper wrote:
>> local_irq_restore() should only be concerned with possibly changing the
>> interrupt flag.  A blind popf could corrupt other system flags.
>> 
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> CC: Keir Fraser <keir@xxxxxxx>
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> ---
>>  xen/include/asm-x86/system.h |    6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>> 
>> diff --git a/xen/include/asm-x86/system.h b/xen/include/asm-x86/system.h
>> index 6ab7d56..cbf0f6a 100644
>> --- a/xen/include/asm-x86/system.h
>> +++ b/xen/include/asm-x86/system.h
>> @@ -159,8 +159,10 @@ 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" );               \
>> +    if ( x & X86_EFLAGS_IF )                                     \
>> +        local_irq_enable();                                      \
>> +    else                                                         \
>> +        local_irq_disable();                                     \
>>  })
> 
> This adds a branch in a potentially hot path.
> 
> Is the local_irq_disable() needed? Interrupts should already be disabled
> on entry.

If that is always true, and so we get rid of the else, then
local_irq_restore() should ASSERT it on entry.

 -- Keir

> David



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