[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 1/3] x86/irq: local_irq_restore() should not blindly popf
>>> 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. To deal with the two branches the code adds I'd rather recommend merging current and to-be-restored flags - the necessary "and" and "or" are quite likely faster than any mis-predicted branch (and perhaps as fast as a correctly predicted one). Jan _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
|
Lists.xenproject.org is hosted with RackSpace, monitoring our |