From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keir Fraser Subject: Re: [PATCH 1/3] x86/irq: local_irq_restore() should not blindly popf Date: Mon, 21 Oct 2013 15:09:23 +0100 Message-ID: References: <526532FB.9010606@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <526532FB.9010606@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: David Vrabel , Andrew Cooper Cc: Jan Beulich , Xen-devel List-Id: xen-devel@lists.xenproject.org On 21/10/2013 14:58, "David Vrabel" 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 >> CC: Keir Fraser >> CC: Jan Beulich >> --- >> 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