From mboxrd@z Thu Jan 1 00:00:00 1970 From: Igor Mammedov Subject: Re: [PATCH] xen: x86_32: do not enable iterrupts when returning from exception in interrupt context Date: Thu, 01 Sep 2011 10:19:03 +0200 Message-ID: <4E5F3FF7.8010805@redhat.com> References: <20110829194633.GB16530@dumpdata.com> <1314834438-3181-1-git-send-email-imammedo@redhat.com> <4E5EB794.7050909@goop.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4E5EB794.7050909@goop.org> Sender: linux-kernel-owner@vger.kernel.org To: Jeremy Fitzhardinge Cc: linux-kernel@vger.kernel.org, xen-devel@lists.xensource.com, konrad.wilk@oracle.com List-Id: xen-devel@lists.xenproject.org On 09/01/2011 12:37 AM, Jeremy Fitzhardinge wrote: > On 08/31/2011 04:47 PM, Igor Mammedov wrote: >> If vmalloc page_fault happens inside of interrupt handler with interrupts >> disabled then on exit path from exception handler when there is no pending >> interrupts, the following code (arch/x86/xen/xen-asm_32.S:112): >> >> cmpw $0x0001, XEN_vcpu_info_pending(%eax) >> sete XEN_vcpu_info_mask(%eax) >> >> will enable interrupts even if they has been previously disabled according to >> eflags from the bounce frame (arch/x86/xen/xen-asm_32.S:99) >> >> testb $X86_EFLAGS_IF>>8, 8+1+ESP_OFFSET(%esp) >> setz XEN_vcpu_info_mask(%eax) >> >> Solution is in setting XEN_vcpu_info_mask only when it should be set >> according to >> cmpw $0x0001, XEN_vcpu_info_pending(%eax) >> but not clearing it if there isn't any pending events. > > Wow, that's a great find. I guess it shows how rarely we end up doing > an exception return with interrupts disabled, since that's been there > since, erm, 2.6.23? > > But this could definitely explain some bugs where interrupts became > unexpectedly re-enabled. Were you tracking one down when you found this? > >> Signed-off-by: Igor Mammedov >> --- >> arch/x86/xen/xen-asm_32.S | 6 +++++- >> 1 files changed, 5 insertions(+), 1 deletions(-) >> >> diff --git a/arch/x86/xen/xen-asm_32.S b/arch/x86/xen/xen-asm_32.S >> index 22a2093..313dca7 100644 >> --- a/arch/x86/xen/xen-asm_32.S >> +++ b/arch/x86/xen/xen-asm_32.S >> @@ -113,10 +113,14 @@ xen_iret_start_crit: >> >> /* >> * If there's something pending, mask events again so we can >> - * jump back into xen_hypervisor_callback >> + * jump back into xen_hypervisor_callback. Otherwise do not >> + * touch XEN_vcpu_info_mask. >> */ >> + jne ignore_vcpu_info_mask >> sete XEN_vcpu_info_mask(%eax) >> >> +ignore_vcpu_info_mask: >> + > > This should be: > > jne 1f > movb $1, XEN_vcpu_info_mask(%eax) > > 1: popl %eax > > > There's no point in using sete if we're already using a conditional jump > to avoid the write, and it's better to use local labels for little > control flow changes like this. > > Thanks, > J Jeremy, Thanks for review, I'll re-post it soon.