[PATCH] x86/kexec: Fix crash on transition to a 32bit kernel on AMD hardware

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The `ljmp *mem` instruction is (famously?) not binary compatible between Intel
and AMD CPUS.  The AMD-compatible version would require .long to be .quad in
the second hunk.

Switch to using lretq, which is compatible between Intel and AMD, as well as
being less logic overall.

Fixes: 5a82d5cf352d ("kexec: extend hypercall with improved load/unload ops")
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Ian Jackson <iwj@xxxxxxxxxxxxxx>

For 4.16.  This is a bugfix for rare (so rare it has probably never been
exercised) but plain-broken usecase.

One argument against taking it says that this has been broken for 8 years
already, so what's a few extra weeks.  Another is that this patch is only
compile tested because I don't have a suitable setup to repro, nor the time to
try organising one.

On the other hand, I specifically used the point of binary incompatibility to
persuade Intel to drop Call Gates out of the architecture in the forthcoming
FRED spec.

The lretq pattern used here matches x86_32_switch() in
xen/arch/x86/boot/head.S, and this codepath is executed on every MB2+EFI
xen.gz boot, which from XenServer alone is a very wide set of testing.
---
 xen/arch/x86/x86_64/kexec_reloc.S | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/x86_64/kexec_reloc.S 
b/xen/arch/x86/x86_64/kexec_reloc.S
index d488d127cfb9..a93f92b19248 100644
--- a/xen/arch/x86/x86_64/kexec_reloc.S
+++ b/xen/arch/x86/x86_64/kexec_reloc.S
@@ -86,12 +86,11 @@ call_32_bit:
         movq    %rax, (compat_mode_gdt_desc + 2)(%rip)
         lgdt    compat_mode_gdt_desc(%rip)
 
-        /* Relocate compatibility mode entry point address. */
-        leal    compatibility_mode(%rip), %eax
-        movl    %eax, compatibility_mode_far(%rip)
-
         /* Enter compatibility mode. */
-        ljmp    *compatibility_mode_far(%rip)
+        lea     compatibility_mode(%rip), %rax
+        push    $0x10
+        push    %rax
+        lretq
 
 relocate_pages:
         /* %rdi - indirection page maddr */
@@ -171,10 +170,6 @@ compatibility_mode:
         ud2
 
         .align 4
-compatibility_mode_far:
-        .long 0x00000000             /* set in call_32_bit above */
-        .word 0x0010
-
 compat_mode_gdt_desc:
         .word .Lcompat_mode_gdt_end - compat_mode_gdt -1
         .quad 0x0000000000000000     /* set in call_32_bit above */
-- 
2.11.0