Re: guest crash in wrmsr_hypervisor_regs if hypercall page is paged out

[Olaf Hering <olaf@aepfle.de>]

On Thu, May 02, Tim Deegan wrote:

> At 20:19 +0200 on 30 Apr (1367353157), Olaf Hering wrote:
> > 
> > With current xen-unstable I see this guest crash if the gfn 169ff is
> > paged out. The xenpaging -v output shows that 169ff is populated, but
> > appearently wrmsr_hypervisor_regs does not like the resulting mfn?!
> 
> Looks that way:
> 
> > (XEN) HVM10: Allocated Xen hypercall page at 169ff000
> > (XEN) traps.c:654:d10 Bad GMFN 169ff (MFN 3e900000000) to MSR 40000000
> 
> That MFN looks like garbage, so I'm guessing that 'page' was null, i.e.
> get_page_from_gfn() returned NULL.  I guess you'll need to instrument it
> up to figure out why.  At least the GFN is a predictable constant which
> should make it easier to add debugging printout for just this case.

The GMFN has p2m_t p2m_ram_paged, so the mfn is -1.

Its not clear to me, how should wrmsr_hypervisor_regs handle a paged
gfn? I was under the impression that get_page_from_gfn would wait until
the gfn is paged-in again.

Olaf