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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

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.

David

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