[Xen-devel] Re: [PATCH] xen: x86_32: do not enable iterrupts when returning from exception in interrupt context

[Igor Mammedov <imammedo@xxxxxxxxxx>]

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<imammedo@xxxxxxxxxx>
> > ---
> >   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.

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