[RFC] x86/kvm/emulate: Avoid RET for fastops

Peter Zijlstra
2023-11-12T20:12:34Z

Hi,

Inspired by the likes of ba5ca5e5e6a1 ("x86/retpoline: Don't clobber
RFLAGS during srso_safe_ret()") I had it on my TODO to look at this,
because the call-depth-tracking rethunk definitely also clobbers flags
and that's a ton harder to fix.

Looking at this recently I noticed that there's really only one callsite
(twice, the testcc thing is basically separate from the rest of the
fastop stuff) and thus CALL+RET is totally silly, we can JMP+JMP.

The below implements this, and aside from objtool going apeshit (it
fails to recognise the fastop JMP_NOSPEC as a jump-table and instead
classifies it as a tail-call), it actually builds and the asm looks
good sensible enough.

I've not yet figured out how to test this stuff, but does something like
this look sane to you guys?

Given that rethunks are quite fat and slow, this could be sold as a
performance optimization I suppose.

---

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index f93e9b96927a..2cd3b5a46e7a 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -412,6 +412,17 @@ static inline void call_depth_return_thunk(void) {}
"call *%[thunk_target]\n", \
X86_FEATURE_RETPOLINE_LFENCE)

+# define JMP_NOSPEC \
+ ALTERNATIVE_2( \
+ ANNOTATE_RETPOLINE_SAFE \
+ "jmp *%[thunk_target]\n", \
+ "jmp __x86_indirect_thunk_%V[thunk_target]\n", \
+ X86_FEATURE_RETPOLINE, \
+ "lfence;\n" \
+ ANNOTATE_RETPOLINE_SAFE \
+ "jmp *%[thunk_target]\n", \
+ X86_FEATURE_RETPOLINE_LFENCE)
+
# define THUNK_TARGET(addr) [thunk_target] "r" (addr)

#else /* CONFIG_X86_32 */
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 2673cd5c46cb..9aae15d223a8 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -294,7 +294,6 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);

#define __FOP_FUNC(name) \
".align " __stringify(FASTOP_SIZE) " \n\t" \
- ".type " name ", @function \n\t" \
name ":\n\t" \
ASM_ENDBR \
IBT_NOSEAL(name)
@@ -302,12 +301,15 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
#define FOP_FUNC(name) \
__FOP_FUNC(#name)

-#define __FOP_RET(name) \
- "11: " ASM_RET \
+#define __FOP_JMP(name, label) \
+ "11: jmp " label " ; int3 \n\t" \
".size " name ", .-" name "\n\t"

-#define FOP_RET(name) \
- __FOP_RET(#name)
+#define FOP_JMP(name, label) \
+ __FOP_JMP(#name, #label)
+
+#define __FOP_RET(name) \
+ __FOP_JMP(name, "fastop_return")

#define __FOP_START(op, align) \
extern void em_##op(struct fastop *fake); \
@@ -420,7 +422,7 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
#define FOP_SETCC(op) \
FOP_FUNC(op) \
#op " %al \n\t" \
- FOP_RET(op)
+ FOP_JMP(op, setcc_return)

FOP_START(setcc)
FOP_SETCC(seto)
@@ -444,7 +446,7 @@ FOP_END;
FOP_START(salc)
FOP_FUNC(salc)
"pushf; sbb %al, %al; popf \n\t"
-FOP_RET(salc)
+FOP_JMP(salc, fastop_return)
FOP_END;

/*
@@ -1061,13 +1063,13 @@ static int em_bsr_c(struct x86_emulate_ctxt *ctxt)
return fastop(ctxt, em_bsr);
}

-static __always_inline u8 test_cc(unsigned int condition, unsigned long flags)
+static noinline u8 test_cc(unsigned int condition, unsigned long flags)
{
u8 rc;
void (*fop)(void) = (void *)em_setcc + FASTOP_SIZE * (condition & 0xf);

flags = (flags & EFLAGS_MASK) | X86_EFLAGS_IF;
- asm("push %[flags]; popf; " CALL_NOSPEC
+ asm("push %[flags]; popf; " JMP_NOSPEC "; setcc_return:"
: "=a"(rc) : [thunk_target]"r"(fop), [flags]"r"(flags));
return rc;
}
@@ -5101,14 +5103,14 @@ static void fetch_possible_mmx_operand(struct operand *op)
kvm_read_mmx_reg(op->addr.mm, &op->mm_val);
}

-static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop)
+static noinline int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop)
{
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_NOSPEC " ; pushf; pop %[flags]\n"
+ asm("push %[flags]; popf; " JMP_NOSPEC " ; fastop_return: ; pushf; pop %[flags]\n"
: "+a"(ctxt->dst.val), "+d"(ctxt->src.val), [flags]"+D"(flags),
[thunk_target]"+S"(fop), ASM_CALL_CONSTRAINT
: "c"(ctxt->src2.val));