[PATCH v2 2/2] x86/trampoline: load the GDT located in the trampoline page

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

When booting the BSP the portion of the code executed from the
trampoline page will be using the GDT located in the hypervisor
.text.head section rather than the GDT located in the relocated
trampoline page.

If skip_realmode is not set the GDT located in the trampoline page
will be loaded after having executed the BIOS call, otherwise the GDT
from .text.head will be used for all the protected mode trampoline
code execution.

Note that both gdt_boot_descr and gdt_48 contain the same entries, but
the former is located inside the hypervisor .text section, while the
later lives in the relocated trampoline page.

This is not harmful as-is, as both GDTs contain the same entries, but
for consistency with the APs switch the BSP trampoline code to also
use the GDT on the relocated trampoline page.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
Changes since v1:
 - Reword comment.
---
 xen/arch/x86/boot/trampoline.S | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/xen/arch/x86/boot/trampoline.S b/xen/arch/x86/boot/trampoline.S
index cdecf949b410..c6005fa33d1f 100644
--- a/xen/arch/x86/boot/trampoline.S
+++ b/xen/arch/x86/boot/trampoline.S
@@ -164,6 +164,9 @@ GLOBAL(trampoline_cpu_started)
 
         .code32
 trampoline_boot_cpu_entry:
+        /* Switch to relocated trampoline GDT. */
+        lgdt    bootsym_rel(gdt_48, 4)
+
         cmpb    $0,bootsym_rel(skip_realmode,5)
         jnz     .Lskip_realmode
 
-- 
2.40.0