[Xen-devel] "Iomem mapping not permitted" during windows crash dump under GPLPV

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

I've recently noticed that my windows crash dumps fail at around 40-50%
under GPLPV. 'xm dmesg' shows the following:

(XEN) grant_table.c:350:d0 Iomem mapping not permitted ffffffffffffffff
(domain 865)

At first I thought that the cause was just a bug in my grant ref code
but it just occurred to me that this could be happening when Windows
tries to write out the physical page I have mapped for some other xen
function. grant_table.c:350 is around here:

if ( !iomem_access_permitted(rd, frame, frame) )
{
  gdprintk(XENLOG_WARNING,
           "Iomem mapping not permitted %lx (domain %d)\n",
           frame, rd->domain_id);
  rc = GNTST_general_error;
  goto undo_out;
}

So is that likely to be the cause? I haven't yet checked the pfn that is
failing against the pages I've mapped for various things but the number
seems plausible. It used to work... is that check new under 3.4.1-ish?

And whats the solution? At this stage the only think I can think of is
that it might be reasonable to ignore a small amount of failed writes
during a crash dump... I haven't yet researched if there is a way to
tell windows not to write out a given page of memory when doing a crash
dump.

Thanks

James

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