Re: [RFC PATCH 3/4] x86/asm: Make alternative macro interfaces more clear and consistent

From: Josh Poimboeuf
Date: Fri Sep 15 2017 - 19:29:33 EST

On Fri, Sep 15, 2017 at 11:01:19AM -0700, Linus Torvalds wrote:
> On Fri, Sep 15, 2017 at 9:53 AM, Andrey Ryabinin
> <aryabinin@xxxxxxxxxxxxx> wrote:
> >
> > I'm not so sure that this is disabled optimization. I assume that global rsp makes
> > changes something in gcc's register allocation logic, or something like that leading
> > to subtle changes in generated code.
> >
> > But what I recently find out, is that this "regression" sometimes is actually improvement in .text size.
> > It all depends on .config, e.g:
>
> Oh, that would be lovely and solve all the issues.
>
> And looking at the code generation differences for one file
> (kernel/futex.c) and one single config (my default config), the thing
> that the global stack register seems to change is that it moves some
> code - particularly completely unrelated inline asm code - inside the
> region protected by frame pointers.
>
> There are a few register allocation changes too, but they didn't seem
> to make code worse, and I think they were just "incidental" from code
> movement. And most code movement really seemed to be around inline
> asms, I wonder if the gcc logic simply is something like "if the
> stack pointer is visible as a register, don't move any inline asm
> across a frame setup".
>
> In fact, on that one file and one configuration, the resulting
> assembler file had three fewer lines of code with that global stack
> register declaration than with the local one.
>
> So at least from just that one case, I can back up Andrey's
> observation: it's not that the code gets worse, it just is slightly
> different. Sometimes it's better.
>
> So maybe that simple patch to just make the stack pointer be a global
> register declaration really is the fix for this issue.
>
> It's not *pretty*, and I'd much rather just see some explicit way for
> us to say "this asm wants the frame to be set up", but of the
> alternatives we've seen, maybe it's the right thing to do?

Ok, here's the (compile tested) patch in case anybody wants to try it
out.


diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 1b020381ab38..c096624137ae 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -218,10 +218,9 @@ static inline int alternatives_text_reserved(void *start, void *end)
#define alternative_call_2(oldfunc, newfunc1, feature1, newfunc2, feature2, \
output, input...) \
{ \
- register void *__sp asm(_ASM_SP); \
asm volatile (ALTERNATIVE_2("call %P[old]", "call %P[new1]", feature1,\
"call %P[new2]", feature2) \
- : output, "+r" (__sp) \
+ : output, ASM_CALL_CONSTRAINT \
: [old] "i" (oldfunc), [new1] "i" (newfunc1), \
[new2] "i" (newfunc2), ## input); \
}
diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
index 676ee5807d86..3914cdf0e488 100644
--- a/arch/x86/include/asm/asm.h
+++ b/arch/x86/include/asm/asm.h
@@ -132,4 +132,9 @@
/* For C file, we already have NOKPROBE_SYMBOL macro */
#endif

+#ifndef __ASSEMBLY__
+register void *__asm_call_sp asm(_ASM_SP);
+#define ASM_CALL_CONSTRAINT "+r" (__asm_call_sp)
+#endif
+
#endif /* _ASM_X86_ASM_H */
diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index 63cc96f064dc..647e2f1d4cb6 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -179,7 +179,6 @@ static inline u64 hv_do_hypercall(u64 control, void *input, void *output)
u64 input_address = input ? virt_to_phys(input) : 0;
u64 output_address = output ? virt_to_phys(output) : 0;
u64 hv_status;
- register void *__sp asm(_ASM_SP);

#ifdef CONFIG_X86_64
if (!hv_hypercall_pg)
@@ -187,7 +186,7 @@ static inline u64 hv_do_hypercall(u64 control, void *input, void *output)

__asm__ __volatile__("mov %4, %%r8\n"
"call *%5"
- : "=a" (hv_status), "+r" (__sp),
+ : "=a" (hv_status), ASM_CALL_CONSTRAINT,
"+c" (control), "+d" (input_address)
: "r" (output_address), "m" (hv_hypercall_pg)
: "cc", "memory", "r8", "r9", "r10", "r11");
@@ -202,7 +201,7 @@ static inline u64 hv_do_hypercall(u64 control, void *input, void *output)

__asm__ __volatile__("call *%7"
: "=A" (hv_status),
- "+c" (input_address_lo), "+r" (__sp)
+ "+c" (input_address_lo), ASM_CALL_CONSTRAINT,
: "A" (control),
"b" (input_address_hi),
"D"(output_address_hi), "S"(output_address_lo),
@@ -224,12 +223,11 @@ static inline u64 hv_do_hypercall(u64 control, void *input, void *output)
static inline u64 hv_do_fast_hypercall8(u16 code, u64 input1)
{
u64 hv_status, control = (u64)code | HV_HYPERCALL_FAST_BIT;
- register void *__sp asm(_ASM_SP);

#ifdef CONFIG_X86_64
{
__asm__ __volatile__("call *%4"
- : "=a" (hv_status), "+r" (__sp),
+ : "=a" (hv_status), ASM_CALL_CONSTRAINT,
"+c" (control), "+d" (input1)
: "m" (hv_hypercall_pg)
: "cc", "r8", "r9", "r10", "r11");
@@ -242,7 +240,7 @@ static inline u64 hv_do_fast_hypercall8(u16 code, u64 input1)
__asm__ __volatile__ ("call *%5"
: "=A"(hv_status),
"+c"(input1_lo),
- "+r"(__sp)
+ ASM_CALL_CONSTRAINT
: "A" (control),
"b" (input1_hi),
"m" (hv_hypercall_pg)
diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
index 42873edd9f9d..280d94c36dad 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -459,8 +459,8 @@ int paravirt_disable_iospace(void);
*/
#ifdef CONFIG_X86_32
#define PVOP_VCALL_ARGS \
- unsigned long __eax = __eax, __edx = __edx, __ecx = __ecx; \
- register void *__sp asm("esp")
+ unsigned long __eax = __eax, __edx = __edx, __ecx = __ecx;
+
#define PVOP_CALL_ARGS PVOP_VCALL_ARGS

#define PVOP_CALL_ARG1(x) "a" ((unsigned long)(x))
@@ -480,8 +480,8 @@ int paravirt_disable_iospace(void);
/* [re]ax isn't an arg, but the return val */
#define PVOP_VCALL_ARGS \
unsigned long __edi = __edi, __esi = __esi, \
- __edx = __edx, __ecx = __ecx, __eax = __eax; \
- register void *__sp asm("rsp")
+ __edx = __edx, __ecx = __ecx, __eax = __eax;
+
#define PVOP_CALL_ARGS PVOP_VCALL_ARGS

#define PVOP_CALL_ARG1(x) "D" ((unsigned long)(x))
@@ -532,7 +532,7 @@ int paravirt_disable_iospace(void);
asm volatile(pre \
paravirt_alt(PARAVIRT_CALL) \
post \
- : call_clbr, "+r" (__sp) \
+ : call_clbr, ASM_CALL_CONSTRAINT \
: paravirt_type(op), \
paravirt_clobber(clbr), \
##__VA_ARGS__ \
@@ -542,7 +542,7 @@ int paravirt_disable_iospace(void);
asm volatile(pre \
paravirt_alt(PARAVIRT_CALL) \
post \
- : call_clbr, "+r" (__sp) \
+ : call_clbr, ASM_CALL_CONSTRAINT \
: paravirt_type(op), \
paravirt_clobber(clbr), \
##__VA_ARGS__ \
@@ -569,7 +569,7 @@ int paravirt_disable_iospace(void);
asm volatile(pre \
paravirt_alt(PARAVIRT_CALL) \
post \
- : call_clbr, "+r" (__sp) \
+ : call_clbr, ASM_CALL_CONSTRAINT \
: paravirt_type(op), \
paravirt_clobber(clbr), \
##__VA_ARGS__ \
diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h
index ec1f3c651150..0bfe1c272db7 100644
--- a/arch/x86/include/asm/preempt.h
+++ b/arch/x86/include/asm/preempt.h
@@ -100,19 +100,14 @@ static __always_inline bool should_resched(int preempt_offset)

#ifdef CONFIG_PREEMPT
extern asmlinkage void ___preempt_schedule(void);
-# define __preempt_schedule() \
-({ \
- register void *__sp asm(_ASM_SP); \
- asm volatile ("call ___preempt_schedule" : "+r"(__sp)); \
-})
+# define __preempt_schedule() \
+ asm volatile ("call ___preempt_schedule" : ASM_CALL_CONSTRAINT);

extern asmlinkage void preempt_schedule(void);
extern asmlinkage void ___preempt_schedule_notrace(void);
-# define __preempt_schedule_notrace() \
-({ \
- register void *__sp asm(_ASM_SP); \
- asm volatile ("call ___preempt_schedule_notrace" : "+r"(__sp)); \
-})
+# define __preempt_schedule_notrace() \
+ asm volatile ("call ___preempt_schedule_notrace" : ASM_CALL_CONSTRAINT);
+
extern asmlinkage void preempt_schedule_notrace(void);
#endif

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 3fa26a61eabc..b390ff76e58f 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -677,8 +677,6 @@ static inline void sync_core(void)
* Like all of Linux's memory ordering operations, this is a
* compiler barrier as well.
*/
- register void *__sp asm(_ASM_SP);
-
#ifdef CONFIG_X86_32
asm volatile (
"pushfl\n\t"
@@ -686,7 +684,7 @@ static inline void sync_core(void)
"pushl $1f\n\t"
"iret\n\t"
"1:"
- : "+r" (__sp) : : "memory");
+ : ASM_CALL_CONSTRAINT : : "memory");
#else
unsigned int tmp;

@@ -703,7 +701,7 @@ static inline void sync_core(void)
"iretq\n\t"
UNWIND_HINT_RESTORE
"1:"
- : "=&r" (tmp), "+r" (__sp) : : "cc", "memory");
+ : "=&r" (tmp), ASM_CALL_CONSTRAINT : : "cc", "memory");
#endif
}

diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
index a34e0d4b957d..7116b7931c7b 100644
--- a/arch/x86/include/asm/rwsem.h
+++ b/arch/x86/include/asm/rwsem.h
@@ -103,7 +103,6 @@ static inline bool __down_read_trylock(struct rw_semaphore *sem)
({ \
long tmp; \
struct rw_semaphore* ret; \
- register void *__sp asm(_ASM_SP); \
\
asm volatile("# beginning down_write\n\t" \
LOCK_PREFIX " xadd %1,(%4)\n\t" \
@@ -114,7 +113,8 @@ static inline bool __down_read_trylock(struct rw_semaphore *sem)
" call " slow_path "\n" \
"1:\n" \
"# ending down_write" \
- : "+m" (sem->count), "=d" (tmp), "=a" (ret), "+r" (__sp) \
+ : "+m" (sem->count), "=d" (tmp), \
+ "=a" (ret), ASM_CALL_CONSTRAINT \
: "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS) \
: "memory", "cc"); \
ret; \
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 184eb9894dae..78e8fcc87d4c 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -166,11 +166,11 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
({ \
int __ret_gu; \
register __inttype(*(ptr)) __val_gu asm("%"_ASM_DX); \
- register void *__sp asm(_ASM_SP); \
__chk_user_ptr(ptr); \
might_fault(); \
asm volatile("call __get_user_%P4" \
- : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp) \
+ : "=a" (__ret_gu), "=r" (__val_gu), \
+ ASM_CALL_CONSTRAINT \
: "0" (ptr), "i" (sizeof(*(ptr)))); \
(x) = (__force __typeof__(*(ptr))) __val_gu; \
__builtin_expect(__ret_gu, 0); \
diff --git a/arch/x86/include/asm/xen/hypercall.h b/arch/x86/include/asm/xen/hypercall.h
index 9606688caa4b..128a1a0b1450 100644
--- a/arch/x86/include/asm/xen/hypercall.h
+++ b/arch/x86/include/asm/xen/hypercall.h
@@ -113,10 +113,9 @@ extern struct { char _entry[32]; } hypercall_page[];
register unsigned long __arg2 asm(__HYPERCALL_ARG2REG) = __arg2; \
register unsigned long __arg3 asm(__HYPERCALL_ARG3REG) = __arg3; \
register unsigned long __arg4 asm(__HYPERCALL_ARG4REG) = __arg4; \
- register unsigned long __arg5 asm(__HYPERCALL_ARG5REG) = __arg5; \
- register void *__sp asm(_ASM_SP);
+ register unsigned long __arg5 asm(__HYPERCALL_ARG5REG) = __arg5;

-#define __HYPERCALL_0PARAM "=r" (__res), "+r" (__sp)
+#define __HYPERCALL_0PARAM "=r" (__res), ASM_CALL_CONSTRAINT
#define __HYPERCALL_1PARAM __HYPERCALL_0PARAM, "+r" (__arg1)
#define __HYPERCALL_2PARAM __HYPERCALL_1PARAM, "+r" (__arg2)
#define __HYPERCALL_3PARAM __HYPERCALL_2PARAM, "+r" (__arg3)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 16bf6655aa85..f23f13403f33 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -5296,7 +5296,6 @@ 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))
@@ -5304,7 +5303,7 @@ static int fastop(struct x86_emulate_ctxt *ctxt, void (*fop)(struct fastop *))

asm("push %[flags]; popf; call *%[fastop]; pushf; pop %[flags]\n"
: "+a"(ctxt->dst.val), "+d"(ctxt->src.val), [flags]"+D"(flags),
- [fastop]"+S"(fop), "+r"(__sp)
+ [fastop]"+S"(fop), ASM_CALL_CONSTRAINT
: "c"(ctxt->src2.val));

ctxt->eflags = (ctxt->eflags & ~EFLAGS_MASK) | (flags & EFLAGS_MASK);
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 699704d4bc9e..efedabe435c9 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -9036,7 +9036,6 @@ static void vmx_complete_atomic_exit(struct vcpu_vmx *vmx)
static void vmx_handle_external_intr(struct kvm_vcpu *vcpu)
{
u32 exit_intr_info = vmcs_read32(VM_EXIT_INTR_INFO);
- register void *__sp asm(_ASM_SP);

if ((exit_intr_info & (INTR_INFO_VALID_MASK | INTR_INFO_INTR_TYPE_MASK))
== (INTR_INFO_VALID_MASK | INTR_TYPE_EXT_INTR)) {
@@ -9065,7 +9064,7 @@ static void vmx_handle_external_intr(struct kvm_vcpu *vcpu)
#ifdef CONFIG_X86_64
[sp]"=&r"(tmp),
#endif
- "+r"(__sp)
+ ASM_CALL_CONSTRAINT
:
[entry]"r"(entry),
[ss]"i"(__KERNEL_DS),
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index b836a7274e12..39567b5c33da 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -806,7 +806,6 @@ no_context(struct pt_regs *regs, unsigned long error_code,
if (is_vmalloc_addr((void *)address) &&
(((unsigned long)tsk->stack - 1 - address < PAGE_SIZE) ||
address - ((unsigned long)tsk->stack + THREAD_SIZE) < PAGE_SIZE)) {
- register void *__sp asm("rsp");
unsigned long stack = this_cpu_read(orig_ist.ist[DOUBLEFAULT_STACK]) - sizeof(void *);
/*
* We're likely to be running with very little stack space
@@ -821,7 +820,7 @@ no_context(struct pt_regs *regs, unsigned long error_code,
asm volatile ("movq %[stack], %%rsp\n\t"
"call handle_stack_overflow\n\t"
"1: jmp 1b"
- : "+r" (__sp)
+ : ASM_CALL_CONSTRAINT
: "D" ("kernel stack overflow (page fault)"),
"S" (regs), "d" (address),
[stack] "rm" (stack));