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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.10.13 at 16:32, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>>>> On 21.10.13 at 16:09, Keir Fraser <keir.xen@xxxxxxxxx> wrote:
>> 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.
> 
> Let's not go that route - the macro is supposed to do what it
> says - restore the prior state of the interrupt flag.

And there is actually an example in the tree that might get
broken if we went that suggested route:
xen/arch/x86/io_apic.c:timer_irq_works(). Albeit I'm not sure - it
looks like the function gets called only in contexts where interrupts
are already enabled, but considering its use of local_irq_enable()
that may not have been the case earlier. In any event - I don't
think we should take the risk of some rarely executed (perhaps
error handling only) path doing something like that.

Jan


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