Re: [Xen-devel] [BUG] Xen vm kernel crash in get_free_entries.

[Matt Wilson <msw@xxxxxxxxx>]

On Sat, Oct 19, 2013 at 01:58:50PM +0200, Sander Eikelenboom wrote:
> 
> Saturday, October 19, 2013, 1:03:17 PM, you wrote:
> 
> > On Sat, 2013-10-19 at 14:51 +0400, Astarta wrote:
> >> On 10/19/2013 03:14 AM, Sander Eikelenboom wrote:
> 
> >> > makes a HVM guest (qemu-xen-traditional) with xen_platform_pci=0 boot 
> >> > again using xl, haven't tested it with xend.
> >> >
> >> Great catch!
> >> I also confirm that 3.11.5 kernel boots just fine after reverting of 
> >> 'correctly initialize grant table version 1' patch.
> 
> > This could just be down to that patch adding some BUG_ONs to catch bad
> > things going on, e.g. the one in gnttab_expand which I think is being
> > hit here.
> 
> > I have a feeling that it is still wrong (but just more benign) to be
> > hitting that call chain in a configuration where there is no platform
> > device driver running. IOW reverting that patch removes the obvious
> > symptom (blowing up) but not the root cause, i.e. the patch is doing its
> > job.
> 
> That was my suspicion too, but at least it seems like some starting point
> of further debugging.
> (and indication of the kernels affected since this commit went to stable as 
> well)
> 
> Since i was still seeing the "Booting PV enabled guest on Xen HVM" is was 
> wondering
> what is supposed to happen when there are some combinations ....

This is the enlightenment code noticing that it's running in a HVM
guest under Xen via the hypervisor cpuid leaf (cpuid leaf
0x40000000).

> xen HVM xen_platform_pci=0 + guest kernel without PV guest support and 
> without xen pv drivers (net + block)

This should work.

> xen HVM xen_platform_pci=0 + guest kernel with PV guest support but without 
> xen pv drivers (net + block)

This should work.

> xen HVM xen_platform_pci=0 + guest kernel with PV guest support and with xen 
> pv drivers (net + block)
> -- This is the configuration that hits the bug described here.

I don't see how this can be expected to work - the PV net and block
devices need the facilities that are initialized by the Xen platform
PCI device to operate. Of course it shouldn't crash either, it should
just use emulated devices instead of xen-netfront/xen-blkfront.

> xen HVM xen_platform_pci=1 + guest kernel without PV guest support and 
> without xen pv drivers (net + block)

This should work.

> xen HVM xen_platform_pci=1 + guest kernel with PV guest support and without 
> xen pv drivers (net + block)

This should work.

> xen HVM xen_platform_pci=1 + guest kernel with PV guest support and with xen 
> pv drivers (net + block)

This should work.

> Booting a guest kernel with PV support as HVM but without using PV doesn't 
> seem possible with a .cfg option ?
> (yes it's a hypothetical option (performance wise), as is running with a 
> guest kernel which supports PV drivers,
>  but not using them with xen_platform_pci=0 .. but it is useful for debugging 
> )

AFAICT the expected behavior would be to for the guest kernel to use
basic enlightenment for CPU operations (hotplug, timers) but no PV IO
support (net + block). But perhaps I'm missing something since you
theoretically don't need the PCI device if you have event channel
callback support in the guest kernel and sufficient support in the
hypervisor.

--msw

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