[Crash-utility] [PATCH] Do not create arch-specific p2m mapping for ELF xen dumps

[Dave Anderson]

----- Original Message -----
> On Fri, 18 Dec 2015 09:33:55 -0500 (EST)
> Dave Anderson <anderson at redhat.com> wrote:
> 
> > ----- Original Message -----
> > > Hello Dave,
> > > 
> > > Let me send you a cleanup/fix for Xen DomU dumps.
> > > 
> > > When opening a Xen DomU dump without auto-translated addresses (e.g.
> > > from an x86_64 PV domain), a mapping between physical and machine
> > > pages is needed. An xc_core dump file does not contain this mapping,
> > > so it must be created by arch-specific code from the dumped data.
> > > 
> > > This (and only this) is the job of the machdep->xendump_p2m_create
> > > callback, which is called if the XC_CORE_P2M_CREATE flag is set. ELF
> > > files contain the mapping in a separate section (".xen_p2m"), so this
> > > arch-specific code is not needed and should not be run.
> > > 
> > > On the other hand, XC_CORE_P2M_CREATE also triggers the initialization
> > > of the elf_index_pfn array. I believe this should be in fact controlled
> > > by XC_CORE_PFN_CREATE, because the flag is not used for anything else.
> > > 
> > > I found this issue thanks to a Xen DomU ELF dump file that contained an
> > > invalid CR3 value, which could not be used to translate kvaddr to maddr.
> > > Without the patch, session initialization fails with these messages:
> > > 
> > > crash: cannot find mfn 0 (0x0) in page index
> > > 
> > > crash: cannot read/find pud page
> > > 
> > > Signed-off-by: Petr Tesarik <ptesarik at suse.com>
> > > 
> > 
> > Hi Petr,
> > 
> > This looks good.  Can you help me come up with a succinct blurb
> > for the changelog?
> > 
> > From my test output, it's now obvious that the gathering of the
> > p2m_frames/p2m_frame_index_list is unnecessary in the case of PV
> > kernel dumps (correct?).
> 
> Not quite. It is still necessary for xc_core files. It is not necessary
> for ELF format files.
> 
> >  And you ran into a situation in which an
> > invalid CR3 in the header caused the (unnecessary) p2m frame-gathering
> > to fail, and as a result the crash session itself was aborted.
> 
> This part is perfect.
> 
> Petr T
> 

Thanks, Petr -- queued for crash-7.1.5:

  https://github.com/crash-utility/crash/commit/c0af94a29dd031ff16efe16e0c2cea6aaf7b4b54

Dave