Re: [Xen-devel] acpidump crashes on some machines

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Aug 23, 2012 at 11:22:29AM +0100, David Vrabel wrote:
> On 23/08/12 11:14, Andre Przywara wrote:
> > On 08/17/2012 10:52 PM, Konrad Rzeszutek Wilk wrote:
> >> On Thu, Jul 26, 2012 at 03:02:58PM +0200, Andre Przywara wrote:
> >>> On 06/30/2012 04:19 AM, Konrad Rzeszutek Wilk wrote:
> >>>
> >>> Konrad, David,
> >>>
> >>> back on track for this issue. Thanks for your input, I could do some
> >>> more debugging (see below for a refresh):
> >>>
> >>> It seems like it affects only the first page of the 1:1 mapping. I
> >>> didn't have an issues with the last PFN or the page behind it (which
> >>> failed properly).
> >>>
> >>> David, thanks for the hint with varying dom0_mem parameter. I
> >>> thought I already checked this, but I did it once again and it
> >>> turned out that it is only an issue if dom0_mem is smaller than the
> >>> ACPI area, which generates a hole in the memory map. So we have
> >>> (simplified)
> >>> * 1:1 mapping to 1 MB
> >>> * normal mapping till dom0_mem
> >>> * unmapped area till ACPI E820 area
> >>> * ACPI E820 1:1 mapping
> >>>
> >>> As far as I could chase it down the 1:1 mapping itself looks OK, I
> >>> couldn't find any off-by-one bugs here. So maybe it is code that
> >>> later on invalidates areas between the normal guest mapping and the
> >>> ACPI mem?
> >>
> >> I think I found it. Can you try this pls [and if you can't find
> >> early_to_phys.. just use the __set_phys_to call]
> > 
> > Yes, that works. At least after a quick test on my test box. Both the 
> > test module and acpidump work as expected. If I replace the "<" in your 
> > patch with the original "<=", I get the warning (and due to the 
> > "continue" it also works).
> 
> Note that the balloon driver could subsequently overwrite the p2m entry.

Hmm, I am not seeing how.. the region that is passed in is right up to
the PFN (I believe). And I did run with this patch over a couple of days
with ballooning up and down. But maybe I missed something?

Let me prep a patch that adds some more checks in the balloon driver
just in case we do hit this.

>  I don't think it is worth redoing the patch to adjust the region passed
> to the balloon driver to avoid this though.
> 
> > I also successfully tested the minimal fix (just replacing <= with <).
> > I will feed it to the testers here to cover more machines.
> > 
> > Do you want to keep the warnings in (which exceed 80 characters, btw)?
> 
> I think we do.
> 
> David

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