[kvm-unit-tests PATCH 01/14] x86/cstart: Don't use MSR_GS_BASE in 32-bit boot code

[Sean Christopherson <seanjc@google.com>]

Load the per-cpu GS.base for 32-bit build by building a temporary GDT
and loading a "real" segment.  Using MSR_GS_BASE is wrong and broken,
it's a 64-bit only MSR and does not exist on 32-bit CPUs.  The current
code works only because 32-bit KVM VMX incorrectly disables interception
of MSR_GS_BASE, and no one runs KVM on an actual 32-bit physical CPU,
i.e. the MSR exists in hardware and so everything "works".

32-bit KVM SVM is not buggy and correctly injects #GP on the WRMSR, i.e.
the tests have never worked on 32-bit SVM.

Fixes: dfe6cb6 ("Add 32 bit smp initialization code")
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 x86/cstart.S | 28 +++++++++++++++++++++++-----
 1 file changed, 23 insertions(+), 5 deletions(-)

diff --git a/x86/cstart.S b/x86/cstart.S
index 489c561..91970a2 100644
--- a/x86/cstart.S
+++ b/x86/cstart.S
@@ -89,13 +89,31 @@ mb_flags = 0x0
 	.long mb_magic, mb_flags, 0 - (mb_magic + mb_flags)
 mb_cmdline = 16
 
-MSR_GS_BASE = 0xc0000101
-
 .macro setup_percpu_area
 	lea -4096(%esp), %eax
-	mov $0, %edx
-	mov $MSR_GS_BASE, %ecx
-	wrmsr
+
+	mov %eax, %edx
+	shl $16, %edx
+	or  $0xffff, %edx
+	mov %edx, 0x10(%eax)
+
+	mov %eax, %edx
+	and $0xff000000, %edx
+	mov %eax, %ecx
+	shr $16, %ecx
+	and $0xff, %ecx
+	or  %ecx, %edx
+	or  $0x00cf9300, %edx
+	mov %edx, 0x14(%eax)
+
+	movw $0x17, 0(%eax)
+	mov %eax, 2(%eax)
+	lgdtl (%eax)
+
+	mov $0x10, %ax
+	mov %ax, %gs
+
+	lgdtl gdt32_descr
 .endm
 
 .macro setup_segments
-- 
2.31.1.498.g6c1eba8ee3d-goog