[Xen-devel] [PATCH v2 3/5] x86/boot: Drop explicit %fs uses

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The trampoline relocation code uses %fs for accessing Xen, and this comes with
an arbitrary 16M limitation.  We could adjust the limit, but the boot code is
a confusing mix of %ds/%esi-based and %fs-based accesses, and the use of %fs
is longer to encode, and incurs an address generation overhead.

Rewrite the logic to use %ds, for better consistency with the surrounding
code, and a marginal performance improvement.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/boot/head.S | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 0137ee99a4..1deeae2f2a 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -718,23 +718,27 @@ trampoline_setup:
         mov     %edx, sym_esi(l2_bootmap)
 
         /* Apply relocations to bootstrap trampoline. */
-        mov     sym_fs(trampoline_phys),%edx
-        mov     $sym_offs(__trampoline_rel_start),%edi
+        mov     sym_esi(trampoline_phys), %edx
+        lea     sym_esi(__trampoline_rel_start), %edi
+        lea     sym_esi(__trampoline_rel_stop), %ecx
 1:
-        mov     %fs:(%edi),%eax
-        add     %edx,%fs:(%edi,%eax)
+        mov     (%edi), %eax
+        add     %edx, (%edi, %eax)
         add     $4,%edi
-        cmp     $sym_offs(__trampoline_rel_stop),%edi
+
+        cmp     %ecx, %edi
         jb      1b
 
         /* Patch in the trampoline segment. */
         shr     $4,%edx
-        mov     $sym_offs(__trampoline_seg_start),%edi
+        lea     sym_esi(__trampoline_seg_start), %edi
+        lea     sym_esi(__trampoline_seg_stop), %ecx
 1:
-        mov     %fs:(%edi),%eax
-        mov     %dx,%fs:(%edi,%eax)
+        mov     (%edi), %eax
+        mov     %dx, (%edi, %eax)
         add     $4,%edi
-        cmp     $sym_offs(__trampoline_seg_stop),%edi
+
+        cmp     %ecx, %edi
         jb      1b
 
         /* Do not parse command line on EFI platform here. */
@@ -760,9 +764,9 @@ trampoline_setup:
         push    %eax
 
         /* Copy bootstrap trampoline to low memory, below 1MB. */
-        mov     $sym_offs(trampoline_start),%esi
+        lea     sym_esi(trampoline_start), %esi
         mov     $((trampoline_end - trampoline_start) / 4),%ecx
-        rep movsl %fs:(%esi),%es:(%edi)
+        rep movsl
 
         /* Jump into the relocated trampoline. */
         lret
-- 
2.11.0


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