Re: [Xen-devel] the ptwr cmpxchg fix

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

Yes, deciding whether to add _PAGE_USER based on Xen version seems the best
way to go. Bear in mind that the ABI bug affects *only* cmpxchg of
pagetables. Any of the following methods of writing a pte with _PAGE_USER
set will also set _PAGE_GLOBAL (unless our software flag _PAGE_GUEST_KERNEL
is also set):
 * update_va_mapping()
 * MMU_NORMAL_PT_UPDATE
 * direct modification of a not-yet-pinned pagetable (the _PAGE_GLOBAL will
be added on each pte when the pagetable becomes pinned)

So, unless you *only* ever update kernel ptes with cmpxchg, you have quite a
nasty problem with older Xen: some updates methods will adjust the l1e,
while direct cmpxchg won't.

Our Linux guests have not had problems because I'm pretty sure we basically
never cmpxchg a kernel pte.

 -- Keir

On 18/10/07 20:58, "John Levon" <levon@xxxxxxxxxxxxxxxxx> wrote:

> 
> I didn't think this through well enough. It's an ABI change.
> 
> The reason Solaris worked at all before was that we didn't remove
> _PAGE_USER for kernel PTEs when compatibility was broken last time
> (3.0.3 I think). Thus the combination of our bug and the hypervisor's
> bug conspired to work.
> 
> Now the hypervisor is fixed, we'll be getting _PAGE_GLOBAL on our kernel
> pages - not a good idea. But we can't just fix Solaris, because other
> hypervisors without the fix will then not be putting _PAGE_USER on
> kernel PTEs - much worse!!
> 
> I think the right thing to do is:
> 
> - finally start the page listing incompatibilities on the Wiki
>   (theoretical or otherwise) [1]
> 
> - fix Solaris to add _PAGE_USER (or PT_USER as we know it) iff we have a
>   'broken' hypervisor. I'm not sure how to do that though, beyond a
>   "if it's our hypervisor, or Xen 3.1.2 or higher" check. BTW it would
>   be nice to see this in 3.1.2
> 
> Does that make sense Keir?
> 
> thanks
> john
> 
> [1]
> 
> This is the list I'm aware of that breaks Solaris domUs:
> 
>     *  Xen 3.1.1 is broken for 64-bit in B75, b76 (6616864).
>       xen-unstable post 2007-10-15 is OK, as is Xen 3.1
>     * Xen 3.0.4 upstream is broken for Solaris domU (doesn't
>       save/restore trap interrupt settings)
>     * pre-3.0.4 doesn't work on 64-bit (changes in PTE handling?)
>     * pre-3.0.4 doesn't work with SMP guests (spurious page fault code)
>     * To quote Jan Beulich:
> 
> Subject: [Xen-devel] c/s 15147 change to struct vcpu_register_vcpu_info
> 
> This changeset changed the layout of the structure, and 3.1 as well as
> 2.6.23 use the old layout, while 3.1.1 uses to new one.
> 
> We don't use this on Solaris yet, however
> 
>     * the fix for cmpxchg and PT_GLOBAL means that newer Solaris
>       versions (or anything cmpxchg'ing a PTE) that correctly don't set
>       PT_USER will break on hypervisors without this changeset:
>       16129:2173fe77dcd2 from xen-unstable
> 
> _______________________________________________
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> http://lists.xensource.com/xen-devel



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