[PATCH] x86/kvm: Add stack frame dependency to fastop() inline asm

From: Josh Poimboeuf
Date: Wed Mar 09 2016 - 14:02:45 EST

The kbuild test robot reported this objtool warning [1]:

arch/x86/kvm/emulate.o: warning: objtool: fastop()+0x69: call without frame pointer save/setup

The issue seems to be caused by CONFIG_PROFILE_ALL_BRANCHES. With that
option, for some reason gcc decides not to create a stack frame in
fastop() before doing the inline asm call, which can result in a bad
stack trace.

Force a stack frame to be created if CONFIG_FRAME_POINTER is enabled by
listing the stack pointer as an output operand for the inline asm

This change has no effect for !CONFIG_PROFILE_ALL_BRANCHES.

[1] https://lists.01.org/pipermail/kbuild-all/2016-March/018249.html

Reported-by: kbuild test robot <fengguang.wu@xxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
arch/x86/kvm/emulate.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 0f62943..a2f24af 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -5110,13 +5110,17 @@ static void fetch_possible_mmx_operand(struct x86_emulate_ctxt *ctxt,

static int fastop(struct x86_emulate_ctxt *ctxt, void (*fop)(struct fastop *))
+ register void *__sp asm(_ASM_SP);
ulong flags = (ctxt->eflags & EFLAGS_MASK) | X86_EFLAGS_IF;
if (!(ctxt->d & ByteOp))
fop += __ffs(ctxt->dst.bytes) * FASTOP_SIZE;
asm("push %[flags]; popf; call *%[fastop]; pushf; pop %[flags]\n"
: "+a"(ctxt->dst.val), "+d"(ctxt->src.val), [flags]"+D"(flags),
- [fastop]"+S"(fop)
+ [fastop]"+S"(fop), "+r"(__sp)
: "c"(ctxt->src2.val));
ctxt->eflags = (ctxt->eflags & ~EFLAGS_MASK) | (flags & EFLAGS_MASK);
if (!fop) /* exception is returned in fop variable */
return emulate_de(ctxt);