x86/entry/compat: Clear RAX high bits on Xen PV SYSENTER

Move the clearing of the high bits of RAX after Xen PV joins the SYSENTER
path so that Xen PV doesn't skip it.

Arguably this code should be deleted instead, but that would belong in the
merge window.

Fixes: ffae641f ("x86/entry/64/compat: Fix Xen PV SYSENTER frame setup")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/9d33b3f3216dcab008070f1c28b6091ae7199969.1593795633.git.luto@kernel.org

arch/x86/entry/entry_64_compat.S

@@ -57,15 +57,6 @@ SYM_CODE_START(entry_SYSENTER_compat)
 
 	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
 
-	/*
-	 * User tracing code (ptrace or signal handlers) might assume that
-	 * the saved RAX contains a 32-bit number when we're invoking a 32-bit
-	 * syscall.  Just in case the high bits are nonzero, zero-extend
-	 * the syscall number.  (This could almost certainly be deleted
-	 * with no ill effects.)
-	 */
-	movl	%eax, %eax
-
 	/* Construct struct pt_regs on stack */
 	pushq	$__USER32_DS		/* pt_regs->ss */
 	pushq	$0			/* pt_regs->sp = 0 (placeholder) */
@@ -80,6 +71,16 @@ SYM_CODE_START(entry_SYSENTER_compat)
 	pushq	$__USER32_CS		/* pt_regs->cs */
 	pushq	$0			/* pt_regs->ip = 0 (placeholder) */
 SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
+
+	/*
+	 * User tracing code (ptrace or signal handlers) might assume that
+	 * the saved RAX contains a 32-bit number when we're invoking a 32-bit
+	 * syscall.  Just in case the high bits are nonzero, zero-extend
+	 * the syscall number.  (This could almost certainly be deleted
+	 * with no ill effects.)
+	 */
+	movl	%eax, %eax
+
 	pushq	%rax			/* pt_regs->orig_ax */
 	pushq	%rdi			/* pt_regs->di */
 	pushq	%rsi			/* pt_regs->si */