Re: [Xen-devel] Fwd: [xen-4.10-testing bisection] complete test-armhf-armhf-xl-arndale

[Julien Grall <julien.grall@xxxxxxx>]

Answering to myself.

On 19/06/2019 10:02, Julien Grall wrote:
> Hi,
>
> On 6/19/19 8:28 AM, Jan Beulich wrote:
> > > > > On 19.06.19 at 09:06, <osstest-admin@xxxxxxxxxxxxxx> wrote:
> > > branch xen-4.10-testing
> > > xenbranch xen-4.10-testing
> > > job test-armhf-armhf-xl-arndale
> > > testid debian-install
> > >
> > > Tree: linux git://xenbits.xen.org/linux-pvops.git
> > > Tree: linuxfirmware git://xenbits.xen.org/osstest/linux-firmware.git
> > > Tree: ovmf git://xenbits.xen.org/osstest/ovmf.git
> > > Tree: qemuu git://xenbits.xen.org/qemu-xen.git
> > > Tree: xen git://xenbits.xen.org/xen.git
> > >
> > > *** Found and reproduced problem changeset ***
> > >
> > >    Bug is in tree:  xen git://xenbits.xen.org/xen.git
> > >    Bug introduced:  702c9146c00d65d1e9c5955335ba002505e97e09
> > >    Bug not present: 52220b5f437a8d03ba108e127e7d717657edf99c
> > >    Last fail repro: http://logs.test-lab.xenproject.org/osstest/logs/138005/
> > >
> > >
> > >    commit 702c9146c00d65d1e9c5955335ba002505e97e09
> > >    Author: Julien Grall <julien.grall@xxxxxxx>
> > >    Date:   Mon Apr 29 15:05:16 2019 +0100
> > >        xen/arm: Add an isb() before reading CNTPCT_EL0 to prevent re-ordering
> > >        Per D8.2.1 in ARM DDI 0487C.a, "a read to CNTPCT_EL0 can occur
> > >        speculatively and out of order relative to other instructions executed
> > >        on the same PE."
> > >        Add an instruction barrier to get accurate number of cycles when
> > >        requested in get_cycles(). For the other users of CNPCT_EL0, replace by
> > >        a call to get_cycles().
> > >        This is part of XSA-295.
> > >        Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> > >        Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > Just in case you didn't notice this coming in.
> I saw it this morning. But I fail to understand how the instruction barrier will 
> result in a timeout during the guest installation...
One thing to keep in mind here is isb() will also carry a compiler barrier. So 
the resulting binary may be different as the compiler may re-order the load.
One possibility is there are a missing data barrier in other part of the code. 
The isb() would expose it.
> The more this only doesn't seem to appear on staging-4.12.
@Stefano, as we know staging-4.12 is working, one way to debug this is to try to 
reproduce in different commit between staging-4.12 with the isb() patch applied.
Hopefully this should test us where the bug was fixed.

Cheers,

--
Julien Grall

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