[Minios-devel] [UNIKRAFT PATCH 1/4] plat/kvm: readibility fixes

[Florian Schmidt <florian.schmidt@xxxxxxxxx>]

* Remove magic numbers from early boot code, replacing it with labels
  and defines.
* Add and rectify comments.

Signed-off-by: Florian Schmidt <florian.schmidt@xxxxxxxxx>
---
 plat/kvm/include/kvm-x86/traps.h |  7 ++++---
 plat/kvm/x86/entry64.S           | 19 +++++++++++++------
 2 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/plat/kvm/include/kvm-x86/traps.h b/plat/kvm/include/kvm-x86/traps.h
index 8210613b..c457bda5 100644
--- a/plat/kvm/include/kvm-x86/traps.h
+++ b/plat/kvm/include/kvm-x86/traps.h
@@ -24,12 +24,12 @@
 /*
  * GDT layout
  *
- * This should be kept consistent with the layout used by the ukvm target (as
- * defined in ukvm/ukvm_cpu_x86_64.h.
+ * This must be kept consistent with the layout used by the kvm entry code (as
+ * defined in plat/kvm/x86/entry64.S)
  */
 #define GDT_DESC_NULL           0
 #define GDT_DESC_CODE           1
-#define GDT_DESC_CODE32         2 /* Used by boot.S on virtio targets */
+#define GDT_DESC_CODE32         2 /* Used by _libkvmplat_start32 */
 #define GDT_DESC_DATA           3
 #define GDT_DESC_TSS_LO         4
 #define GDT_DESC_TSS_HI         5
@@ -39,6 +39,7 @@
 #define GDT_NUM_ENTRIES         6
 
 #define GDT_DESC_CODE_VAL       0x00af99000000ffff
+#define GDT_DESC_CODE32_VAL     0x00cf9b000000ffff
 #define GDT_DESC_DATA_VAL       0x00cf93000000ffff
 
 
diff --git a/plat/kvm/x86/entry64.S b/plat/kvm/x86/entry64.S
index 1ec81364..e15d41e2 100644
--- a/plat/kvm/x86/entry64.S
+++ b/plat/kvm/x86/entry64.S
@@ -79,12 +79,14 @@ ENTRY(_libkvmplat_start32)
        jne nomultiboot
 
        lgdt (gdt64_ptr)
-       pushl $0x0
-       pushw $0x10
+       /* let lret jump just one instruction ahead, but set %cs
+        * to the correct GDT entry while doing that.
+        */
+       pushl $(gdt64_32cs-gdt64)
        pushl $1f
        lret
 
-1:     movl $0x18, %eax
+1:     movl $(gdt64_ds-gdt64), %eax
        movl %eax, %ds
        movl %eax, %es
        movl %eax, %ss
@@ -117,8 +119,10 @@ ENTRY(_libkvmplat_start32)
        orl $X86_CR0_PG, %eax
        movl %eax, %cr0
 
-       /* 5: poetically longjump to longmode */
-       pushw $0x08
+       /* 5: poetically longjump to longmode,
+        *    letting lret set %cs to the correct GDT entry on the way
+        */
+       pushl $(gdt64_64cs-gdt64)
        pushl $_libkvmplat_start64
        lret
 
@@ -145,8 +149,11 @@ END(_libkvmplat_start32)
 .align 64
 gdt64:
        .quad 0x0000000000000000
+gdt64_64cs:
        .quad GDT_DESC_CODE_VAL         /* 64bit CS             */
-       .quad 0x00cf9b000000ffff        /* 32bit CS             */
+gdt64_32cs:
+       .quad GDT_DESC_CODE32_VAL       /* 32bit CS             */
+gdt64_ds:
        .quad GDT_DESC_DATA_VAL         /* DS                   */
        .quad 0x0000000000000000        /* TSS part 1 (via C)   */
        .quad 0x0000000000000000        /* TSS part 2 (via C)   */
-- 
2.21.0


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