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Re: [Xen-devel] [PATCH] Solved the Xen PV/KASLR riddle



On 09/02/2014 09:22 PM, Konrad Rzeszutek Wilk wrote:
On Mon, Sep 01, 2014 at 06:03:06AM +0200, Juergen Gross wrote:
On 08/29/2014 04:55 PM, Konrad Rzeszutek Wilk wrote:
On Fri, Aug 29, 2014 at 03:44:06PM +0100, Jan Beulich wrote:
On 29.08.14 at 16:27, <stefan.bader@xxxxxxxxxxxxx> wrote:
Sure. Btw, someone also contacted me saying they have the same problem
without
changing the layout but having really big initrd (500M). While that feels
like
it should be impossible (if the kernel+initrd+xen stuff has to fix the 512M
kernel image size area then). But if it can happen, then surely it does
cause
mappings to be where the module space starts then.

Since the initrd doesn't really need to be mapped into the (limited)
virtual address space a pv guest starts with, we specifically got

/*
  * Whether or not the guest can deal with being passed an initrd not
  * mapped through its initial page tables.
  */
#define XEN_ELFNOTE_MOD_START_PFN 16

to deal with that situation. The hypervisor side for Dom0 is in place,
and the kernel side works in our (classic) kernels. Whether it got
implemented for DomU meanwhile I don't know; I'm pretty certain
pv-ops kernels don't support it so far.

Correct - Not implemented. Here is what I had mentioned in the past:
(see http://lists.xen.org/archives/html/xen-devel/2014-03/msg00580.html)


XEN_ELFNOTE_INIT_P2M, XEN_ELFNOTE_MOD_START_PFN - I had been looking
     at that but I can't figure out a nice way of implementing this
     without the usage of SPARSEMAP_VMAP virtual addresses - which is how
     the classic Xen does it. But then - I don't know who is using huge PV
     guests - as the PVHVM does a fine job? But then with PVH, now you can
     boot with large amount of memory (1TB?) - so some of these issues
     would go away? Except the 'large ramdisk' as that would eat in the
     MODULES_VADDR I think? Needs more thinking.

.. and then I left it and to my suprise saw on Luis's slides that
Jurgen is going to take a look at that (500GB support).

I have a patch which should do the job. It is based on the classic
kernel patch Jan mentioned above. The system is coming up with it, I
haven't tested it with a huge initrd up to now. My plan was to post the
patch together with the rest of the >500GB support, but I can send it
on it's own if required.

Oooh goodies! I think it makes sense to post it whenever you think
it is in the right state to be posted.

Now that your pvSCSI drivers are in, you have tons of free time
I suspect :-)

Oh yeah. Only one or two lines missing in xl to support it. :-)

I hope to have the >500GB patch ready for testing soon. I'd prefer to
combine this and the large initrd patch in one series, as both need the
same headers to be synced with Xen. In case I'm meeting some serious
issues I'll post the large initrd patch on Friday.

Juergen


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