Re: [ARM] Bash often segfaults in Dom0 with the latest Xen

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

On Wed, 5 Jun 2013, Christoffer Dall wrote:
> On 5 June 2013 10:53, Julien Grall <julien.grall@linaro.org> wrote:
> > On 06/05/2013 06:36 PM, Christoffer Dall wrote:
> >
> >>>
> >>> I'm using the linaro's branch ll_20130528.0, I have only few patches for
> >>> the dts and not yet in linaro tree patches.
> >>>
> >>> I have the same issue with linux 3.9-rc4 with multiple CPUs and I can't
> >>> really go before without carrying many xen patches to try it.
> >>>
> >>> I have tried different configuration with the number of CPUs in Xen
> >>> (pCPU) and linux (vCPU):
> >>>   - 2 pCPU 2 vCPU : segfaulting
> >>>   - 2 pCPU 1 vCPU : working
> >>>   - 1 pCPU 1 vCPU : working
> >>>   - 1 pCPU 2 vCPU : very slow but working
> >>>
> >> 2 pCPU 1 vCPU are you still compiling your dom0 as an SMP kernel, but
> >> only creating 1 vCPU or are you actually compiling the dom0 as UP?
> >
> >
> > Yes. It's same kernel with the same command line (ie without nosmp).
> > I have limited the number of dom0 vcpus with dom0_max_vcpus=1 on xen
> > command line.
> >
> It indicates a bug in Xen then. Curious that it only happens for user
> space in dom0, but perhaps you just haven't seen it in the kernel yet.
> Bash scripts are pretty intensive on page faults so perhaps there's a
> synchronization issue with some of your page fault handlers.
> 
> You could try to touch all the memory inside dom0 (dd to a ramfs for
> example) and then run your bash script and see if the problem still
> occurs, that should point you to whether it's a stage-2 fault handling
> issue, but this is not a fool-proof approach. Maybe Xen can
> pre-allocate all the stage-2 entries?

Xen pre-allocates all the memory for stage-2 entries (no overcommit or
populate on demand by default)