linux-kernel - Re: [Xen-devel] Re: [PATCH v2] xen: x86_32: do not enable iterrupts when returning from exception in interrupt context

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Date:	Fri, 2 Sep 2011 10:47:02 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] Re: [PATCH v2] xen: x86_32: do not enable iterrupts
 when returning from exception in interrupt context

On Fri, Sep 02, 2011 at 04:01:35PM +0200, Igor Mammedov wrote:
> On 09/02/2011 03:40 PM, Konrad Rzeszutek Wilk wrote:
> >On Fri, Sep 02, 2011 at 10:18:23AM +0200, Igor Mammedov wrote:
> >>On 09/01/2011 06:46 PM, Jeremy Fitzhardinge wrote:
> >>>On 09/01/2011 04:46 AM, 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.
> >>>>
> >>>>Reproducer for bug is attached to RHBZ 707552
> >>>>
> >>>>Signed-off-by: Igor Mammedov<imammedo@...hat.com>
> >>>>Signed-off-by: Jeremy Fitzhardinge<jeremy@...p.org>
> >>>
> >>>One nit, this should be acked-by or reviewed-by, not signed-off-by,
> >>>since the patch isn't passing through my hands.
> >>>
> >>>     J
> >>
> >>I'm new to this stuff, would you like me to re-post it?
> >
> >That is OK.  I fixed it up in the git commit. Thanks for finding this one!
> 
> You're welcome!
> I've learned a lot while debugging it. In particular, how to use kvm and qemu's
> gdbstub to debug xen and guest using gdb.

Oh, would you be interested in writting a blog article on xen.org by any chance?
That sounds pretty nifty!
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