[PATCHv6 25/36] vdso: Introduce vdso_static_branch_unlikely()

[Dmitry Safonov]

From: Andrei Vagin <avagin at gmail.com>

As it has been discussed on timens RFC, adding a new conditional branch
`if (inside_time_ns)` on VDSO for all processes is undesirable.

Addressing those problems, there are two versions of VDSO's .so:
for host tasks (without any penalty) and for processes inside of time
namespace with clk_to_ns() that subtracts offsets from host's time.

Introduce vdso_static_branch_unlikely(), which is similar to
static_branch_unlikely(); alias it with timens_static_branch_unlikely()
under CONFIG_TIME_NS.

The timens code in vdso will look like this:

   if (timens_static_branch_unlikely()) {
	   clk_to_ns(clk, ts);
   }

The version of vdso which is compiled from sources will never execute
clk_to_ns(). And then we can patch the 'no-op' in the straight-line
codepath with a 'jump' instruction to the out-of-line true branch and
get the timens version of the vdso library.

Signed-off-by: Andrei Vagin <avagin at gmail.com>
Co-developed-by: Dmitry Safonov <dima at arista.com>
Signed-off-by: Dmitry Safonov <dima at arista.com>
---
 arch/x86/include/asm/jump_label.h | 14 ++++++++++++++
 lib/vdso/gettimeofday.c           | 10 ++++++++--
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index 06c3cc22a058..376efb53183b 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -53,6 +53,20 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
 	return true;
 }
 
+static __always_inline bool vdso_static_branch_unlikely(void)
+{
+	asm_volatile_goto("1:\n\t"
+		".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
+		 ".pushsection __jump_table,  \"aw\"\n\t"
+		 "2: .word 1b - 2b, %l[l_yes] - 2b\n\t"
+		 ".popsection\n\t"
+		 : :  :  : l_yes);
+
+	return false;
+l_yes:
+	return true;
+}
+
 #else	/* __ASSEMBLY__ */
 
 .macro STATIC_JUMP_IF_TRUE target, key, def
diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index 8589c66ff3e7..7df8fa6c03fa 100644
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -8,6 +8,7 @@
 #include <linux/kernel.h>
 #include <linux/hrtimer_defs.h>
 #include <linux/timens_offsets.h>
+#include <linux/jump_label.h>
 #include <vdso/datapage.h>
 #include <vdso/helpers.h>
 
@@ -43,6 +44,8 @@ u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult)
 extern u8 timens_page
 	__attribute__((visibility("hidden")));
 
+#define timens_static_branch_unlikely vdso_static_branch_unlikely
+
 notrace static __always_inline void clk_to_ns(clockid_t clk, struct __kernel_timespec *ts)
 {
 	struct timens_offsets *timens = (struct timens_offsets *) &timens_page;
@@ -79,6 +82,7 @@ notrace static __always_inline void clk_to_ns(clockid_t clk, struct __kernel_tim
 }
 #else
 notrace static __always_inline void clk_to_ns(clockid_t clk, struct __kernel_timespec *ts) {}
+notrace static __always_inline bool timens_static_branch_unlikely(void) { return false; }
 #endif
 
 static int do_hres(const struct vdso_data *vd, clockid_t clk,
@@ -108,7 +112,8 @@ static int do_hres(const struct vdso_data *vd, clockid_t clk,
 	ts->tv_sec = sec + __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
 	ts->tv_nsec = ns;
 
-	clk_to_ns(clk, ts);
+	if (timens_static_branch_unlikely())
+		clk_to_ns(clk, ts);
 
 	return 0;
 }
@@ -125,7 +130,8 @@ static void do_coarse(const struct vdso_data *vd, clockid_t clk,
 		ts->tv_nsec = vdso_ts->nsec;
 	} while (unlikely(vdso_read_retry(vd, seq)));
 
-	clk_to_ns(clk, ts);
+	if (timens_static_branch_unlikely())
+		clk_to_ns(clk, ts);
 }
 
 static __maybe_unused int
-- 
2.22.0