[PATCH v2] x86/PV: ignore PAE_MODE ELF note for 64-bit Dom0

[Jan Beulich <jbeulich@xxxxxxxx>]

Besides a printk() the main effect is slight corruption of the start
info magic: While that's meant to be xen-3.0-x86_64, it wrongly ended
up as xen-3.0-x86_64p.

Note that no known users exist that would have developed a dependency on
the bogus magic string. In particular Linux, NetBSD, and mini-os have
been checked.

Fixes: 460060f83d41 ("libelf: use for x86 dom0 builder")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
RFC: While Linux works fine with the adjustment, I'm not entirely
     certain of external tools (crash?) having grown a dependency. It
     may be worth noting that XenoLinux and its forward ports never had
     this ELF note in 64-bit kernels, so in principle it may be
     reasonable to expect that no such dependency exists anywhere.

Prior to "x86/PV32: restore PAE-extended-CR3 logic" that (meaningless
for 64-bit domains) VM-assist could also be engaged, based on the ELF
note's value. I expect that change to go in first, at which point the
description here is going to be correct (in not mentioning this VM-
assist aspect).
---
v2: Extend description.

--- a/xen/arch/x86/pv/dom0_build.c
+++ b/xen/arch/x86/pv/dom0_build.c
@@ -459,8 +459,13 @@ int __init dom0_construct_pv(struct doma
     compat = is_pv_32bit_domain(d);
 
     if ( elf_64bit(&elf) && machine == EM_X86_64 )
+    {
         compatible = true;
 
+        /* Zap meaningless setting which kernels may carry by mistake. */
+        parms.pae = 0;
+    }
+
     if ( elf_msb(&elf) )
         compatible = false;