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

[Juergen Gross]

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.

Juergen

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