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Re: [Xen-devel] pvgrub "Error 9: Unknown boot failure" booting Debian Jessie kernel (Was: Re: [PATCH v5 6/9] libxc: create unmapped initrd in domain builder if supported)

On Tue, 2015-12-01 at 09:53 +0100, Juergen Gross wrote:
> On 01/12/15 09:30, Ian Campbell wrote:
> > On Tue, 2015-12-01 at 08:41 +0100, Juergen Gross wrote:
> > > > > I'm not quite sure what to make of this, in particular I don't
> > > > > see
> > > anything
> > > > > in kexec.c which obviously looks after unmapping the heap or brk
> > > areas.
> > > > 
> > > > Nah, this backtrace shows a normal allocation path while
> > > > uncompressing the kernel image. I'd expect something like that.
> > > > Why shouldn't mini-os make use of pfn 4d81 somewhere?
> > 
> > That pfn ends up right in the middle of the next-kernels vaddr mapping,
> > so at best it indicates some sort of disconnect/overlap between the
> > mini-os memory allocator and the domain-builder memory allocator.
> I don't think so.
> mini-os just allocates single pages and keeps the relation to the
> (future) pfn of that page. The p2m list is adjusted later to move the
> allocated page to that pfn before activating the new kernel.

Ah, I was wondering how it could possibly work so I half expected I must be
missing something.

> > Since it seems to be in the middle of the padding area (which might
> > have been new in ea7c8a3d0e82, I'm not sure, it seems to be more
> > explicit at the least) it occurred to me on the way home last night
> > that maybe we need to unmap the padding area as well.
> We do. The page tables need to be unmapped independently as they
> have been mapped explicitly during setup_pgtables(dom). All the
> mini-os mappings are removed in a loop just after that.

"a loop" is this:
    /* Unmap day0 pages to avoid having a r/w mapping of the future page table 
 Âfor (pfn = 0; pfn < allocated; pfn++)
ÂÂÂÂÂÂÂÂmunmap((void*) pages[pfn], PAGE_SIZE);

In my debugging this extends only to the end of the actual mappings, not to
the end of the padding, e.g. for me it is extending to "Unmap pfns 0 ..
0x4c0f" while the unexpected PT pfn is at 0x4d80 and the padding area
extends toÂpfn 0x5000.

> > I'll try that and your suggested patch below as well once I get to the
> > office this morning.
> Thanks.

The BUG_ON doesn't seem to be triggering. I'm not seeing pfn==0x4d80 going
anywhere near kexec_allocate, the highest isÂ0x4c0f.

Maybe the issue is that the ->allocate hook (==kexec_allocate) isn't called
from xc_dom_alloc_pad?

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