[PATCH] x86/boot: Fix comment about setting up the real mode stack

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

It may have taken a while, but it occurs to me that the mentioned commit fixed
a second problem too.

On entering trampoline_boot_cpu_entry(), %esp points at the trampoline stack,
but in a 32bit flat segment.  It happens to be page aligned.

When dropping into 16bit mode, the stack segment operates on %sp, preserving
the upper bits.  Prior to 1ed76797439e, the top nibble of %sp would depend on
where the trampoline was placed in low memory, and only had a 1/16 chance of
being 0 and therefore operating on the intended stack.

There was a 15/16 chance of using a different page in the trampoline as if it
were the stack.  Therefore, zeroing %esp was correct, but for more reasons
than realised at the time.

Update the comment to explain why zeroing %esp is correct in all cases.  Move
it marginally earlier to when a 16bit stack segment is first loaded.

Fixes: 1ed76797439e ("x86/boot: fix BIOS memory corruption on certain IBM 
systems")
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Daniel P. Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>
CC: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
CC: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
---
 xen/arch/x86/boot/trampoline.S | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/boot/trampoline.S b/xen/arch/x86/boot/trampoline.S
index 924bda37c1b7..f5a1d61280c5 100644
--- a/xen/arch/x86/boot/trampoline.S
+++ b/xen/arch/x86/boot/trampoline.S
@@ -176,6 +176,12 @@ trampoline_boot_cpu_entry:
         mov     %eax,%gs
         mov     %eax,%ss
 
+        /*
+         * The stack is at trampoline_phys + 64k, which for a 16bit stack
+         * segment wants %sp starting at 0.
+         */
+        xor     %esp, %esp
+
         /* Switch to pseudo-rm CS, enter real mode, and flush insn queue. */
         mov     %cr0,%eax
         dec     %eax
@@ -190,8 +196,6 @@ trampoline_boot_cpu_entry:
         mov     %ax,%es
         mov     %ax,%ss
 
-        /* Initialise stack pointer and IDT, and enable irqs. */
-        xor     %esp,%esp
         lidt    bootsym(rm_idt)
         sti
 

base-commit: 41c80496084aa3601230e01c3bc579a0a6d8359a
prerequisite-patch-id: 6eb3b54ccd19effe3a89768e0ec5c7a2496a233a
prerequisite-patch-id: b9c480479c1f4021e9c3fe659811e28bf88f6eca
prerequisite-patch-id: 68f0d0fff4312fb059861efddbef95ddf4635b0e
prerequisite-patch-id: 66902cf11d58181ff63b8ee4efb4078df5828490
-- 
2.39.5