From: Andrei Vagin <avagin@xxxxxxxxx>
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@xxxxxxxxx>
Co-developed-by: Dmitry Safonov <dima@xxxxxxxxxx>
Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
---
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 7525433f8ba4..605bdb92055d 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
