From 22cc94bb97f4c9ae3fcd3d3c751db935c8c594a2 Mon Sep 17 00:00:00 2001
From: Eduardo Habkost <ehabkost@redhat.com>
Date: Fri, 4 Jan 2008 11:54:37 -0200
Subject: [PATCH] xen-64: xen_iret() use iretq directly if returning to kernel-space

Optimization pulled from upstream Xen.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 arch/x86/xen/xen-asm.S    |    5 ++++
 arch/x86/xen/xen-asm_64.S |   49 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+), 0 deletions(-)

diff --git a/arch/x86/xen/xen-asm.S b/arch/x86/xen/xen-asm.S
index 1c3c0ac..c7356c0 100644
--- a/arch/x86/xen/xen-asm.S
+++ b/arch/x86/xen/xen-asm.S
@@ -21,6 +21,11 @@
 #include <asm/asm-hack.h>
 
 #include <xen/interface/xen.h>
+#include <xen/interface/features.h>
+
+#include <asm/calling.h>
+#include <asm/processor-flags.h>
+
 
 #define RELOC(x, v)	.globl x##_reloc; x##_reloc=v
 #define ENDPATCH(x)	.globl x##_end; x##_end=.
diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
index 8f28a91..85c235c 100644
--- a/arch/x86/xen/xen-asm_64.S
+++ b/arch/x86/xen/xen-asm_64.S
@@ -7,7 +7,56 @@ ENTRY(xen_iret_direct)
 	/*FIXME: implement me! */
 	ud2a
 
+//FIXME: move to calling.h?
+#define IRETOFFSET RIP
+
 ENTRY(xen_iret)
+	/* Are we returning to kernel space? */
+	testb $3,CS-IRETOFFSET(%rsp)
+	jnz xen_iret_slow
+
+#if 0
+	/*FIXME: handle NMI callbacks */
+	testl $NMI_MASK,EFLAGS-IRETOFFSET(%rsp)
+	jnz xen_iret_slow
+#endif
+
+	/* If we are going to restore interrupts, we need to
+	 * return through the hypervisor, too.
+	 *
+	 * We may set evtchn_upcall_mask ourselves, but things get complicated
+	 * because we would need to avoid a stack overflow if we get
+	 * interrupts between upcall_mask being cleared and iret being
+	 * executed.
+	 */
+	testl $X86_EFLAGS_IF,EFLAGS-IRETOFFSET(%rsp)
+	jnz xen_iret_slow
+
+
+
+	/* Returning to kernel (RING0).
+	 * Use direct iret to kernel space after correcting CS and SS
+	 */
+
+	cmpb  $0,(xen_features+XENFEAT_supervisor_mode_kernel)(%rip)
+	/* supervisor_mode_kernel -> no need to correct CS and SS */
+	jne   xen_iretq
+
+	/* Direct iret to kernel space. Correct CS and SS. */
+	orl   $3,CS-IRETOFFSET(%rsp)
+	orl   $3,SS-IRETOFFSET(%rsp)
+xen_iretq:
+	iretq
+
+xen_iret_slow:
+	/* Slow iret via hypervisor */
+
+#if 0
+	/*FIXME: handle NMI callbacks */
+	andl  $~NMI_MASK, 2*8(%rsp)
+#endif
+
+	/* Xen has an additional value in the stack for iret */
 	/*FIXME: the VGCF_in_syscall flag need to be handled here */
 	pushq $0
 	jmp hypercall_page + (__HYPERVISOR_iret * 32)
-- 
1.5.4.1