[PATCH v2 1/2] arm64: Refactor vDSO time functions
Kevin Brodsky
kevin.brodsky at arm.com
Tue Jul 12 03:23:59 PDT 2016
Time functions are directly implemented in assembly in arm64, and it
is desirable to keep it this way for performance reasons (everything
fits in registers, so that the stack is not used at all). However, the
current implementation is quite difficult to read and understand (even
considering it's assembly). Additionally, due to the structure of
__kernel_clock_gettime, which heavily uses conditional branches to
share code between the different clocks, it is difficult to support a
new clock without making the branches even harder to follow.
This commit completely refactors the structure of clock_gettime (and
gettimeofday along the way) while keeping exactly the same algorithms.
We no longer try to share code; instead, macros provide common
operations. This new approach comes with a number of advantages:
- In clock_gettime, clock implementations are no longer interspersed,
making them much more readable. Additionally, macros only use
registers passed as arguments or reserved with .req, this way it is
easy to make sure that registers are properly allocated. To avoid a
large number of branches in a given execution path, a jump table is
used; a normal execution uses 3 unconditional branches.
- __do_get_tspec has been replaced with 2 macros (get_ts_clock_mono,
get_clock_shifted_nsec) and explicit loading of data from the vDSO
page. Consequently, clock_gettime and gettimeofday are now leaf
functions, and saving x30 (lr) is no longer necessary.
- Variables protected by tb_seq_count are now loaded all at once,
allowing to merge the seqcnt_read macro into seqcnt_check.
- For CLOCK_REALTIME_COARSE, removed an unused load of the wall to
monotonic timespec.
- For CLOCK_MONOTONIC_COARSE, removed a few shift instructions.
Obviously, the downside of sharing less code is an increase in code
size. However since the vDSO has its own code page, this does not
really matter, as long as the size of the DSO remains below 4 kB. For
now this should be all right:
Before After
vdso.so size (B) 2776 3000
Signed-off-by: Kevin Brodsky <kevin.brodsky at arm.com>
Reviewed-by: Dave Martin <dave.martin at arm.com>
Acked-by: Will Deacon <will.deacon at arm.com>
---
arch/arm64/kernel/vdso/gettimeofday.S | 298 ++++++++++++++++++++--------------
1 file changed, 172 insertions(+), 126 deletions(-)
diff --git a/arch/arm64/kernel/vdso/gettimeofday.S b/arch/arm64/kernel/vdso/gettimeofday.S
index efa79e8d4196..c06919ee29e1 100644
--- a/arch/arm64/kernel/vdso/gettimeofday.S
+++ b/arch/arm64/kernel/vdso/gettimeofday.S
@@ -26,24 +26,100 @@
#define NSEC_PER_SEC_HI16 0x3b9a
vdso_data .req x6
-use_syscall .req w7
-seqcnt .req w8
+seqcnt .req w7
+w_tmp .req w8
+x_tmp .req x8
+
+/*
+ * Conventions for macro arguments:
+ * - An argument is write-only if its name starts with "res".
+ * - All other arguments are read-only, unless otherwise specified.
+ */
.macro seqcnt_acquire
9999: ldr seqcnt, [vdso_data, #VDSO_TB_SEQ_COUNT]
tbnz seqcnt, #0, 9999b
dmb ishld
- ldr use_syscall, [vdso_data, #VDSO_USE_SYSCALL]
.endm
- .macro seqcnt_read, cnt
+ .macro seqcnt_check fail
dmb ishld
- ldr \cnt, [vdso_data, #VDSO_TB_SEQ_COUNT]
+ ldr w_tmp, [vdso_data, #VDSO_TB_SEQ_COUNT]
+ cmp w_tmp, seqcnt
+ b.ne \fail
.endm
- .macro seqcnt_check, cnt, fail
- cmp \cnt, seqcnt
- b.ne \fail
+ .macro syscall_check fail
+ ldr w_tmp, [vdso_data, #VDSO_USE_SYSCALL]
+ cbnz w_tmp, \fail
+ .endm
+
+ .macro get_nsec_per_sec res
+ mov \res, #NSEC_PER_SEC_LO16
+ movk \res, #NSEC_PER_SEC_HI16, lsl #16
+ .endm
+
+ /*
+ * Returns the clock delta, in nanoseconds left-shifted by the clock
+ * shift.
+ */
+ .macro get_clock_shifted_nsec res, cycle_last, mult
+ /* Read the virtual counter. */
+ isb
+ mrs x_tmp, cntvct_el0
+ /* Calculate cycle delta and convert to ns. */
+ sub \res, x_tmp, \cycle_last
+ /* We can only guarantee 56 bits of precision. */
+ movn x_tmp, #0xff00, lsl #48
+ and \res, x_tmp, \res
+ mul \res, \res, \mult
+ .endm
+
+ /*
+ * Returns in res_{sec,nsec} the REALTIME timespec, based on the
+ * "wall time" (xtime) and the clock_mono delta.
+ */
+ .macro get_ts_realtime res_sec, res_nsec, \
+ clock_nsec, xtime_sec, xtime_nsec, nsec_to_sec
+ add \res_nsec, \clock_nsec, \xtime_nsec
+ udiv x_tmp, \res_nsec, \nsec_to_sec
+ add \res_sec, \xtime_sec, x_tmp
+ msub \res_nsec, x_tmp, \nsec_to_sec, \res_nsec
+ .endm
+
+ /* sec and nsec are modified in place. */
+ .macro add_ts sec, nsec, ts_sec, ts_nsec, nsec_to_sec
+ /* Add timespec. */
+ add \sec, \sec, \ts_sec
+ add \nsec, \nsec, \ts_nsec
+
+ /* Normalise the new timespec. */
+ cmp \nsec, \nsec_to_sec
+ b.lt 9999f
+ sub \nsec, \nsec, \nsec_to_sec
+ add \sec, \sec, #1
+9999:
+ cmp \nsec, #0
+ b.ge 9998f
+ add \nsec, \nsec, \nsec_to_sec
+ sub \sec, \sec, #1
+9998:
+ .endm
+
+ .macro clock_gettime_return, shift=0
+ .if \shift == 1
+ lsr x11, x11, x12
+ .endif
+ stp x10, x11, [x1, #TSPEC_TV_SEC]
+ mov x0, xzr
+ ret
+ .endm
+
+ .macro jump_slot jumptable, index, label
+ .if (. - \jumptable) != 4 * (\index)
+ .error "Jump slot index mismatch"
+ .endif
+ b \label
.endm
.text
@@ -51,18 +127,24 @@ seqcnt .req w8
/* int __kernel_gettimeofday(struct timeval *tv, struct timezone *tz); */
ENTRY(__kernel_gettimeofday)
.cfi_startproc
- mov x2, x30
- .cfi_register x30, x2
-
- /* Acquire the sequence counter and get the timespec. */
adr vdso_data, _vdso_data
-1: seqcnt_acquire
- cbnz use_syscall, 4f
-
/* If tv is NULL, skip to the timezone code. */
cbz x0, 2f
- bl __do_get_tspec
- seqcnt_check w9, 1b
+
+ /* Compute the time of day. */
+1: seqcnt_acquire
+ syscall_check fail=4f
+ ldr x10, [vdso_data, #VDSO_CS_CYCLE_LAST]
+ ldp w11, w12, [vdso_data, #VDSO_CS_MULT]
+ ldp x13, x14, [vdso_data, #VDSO_XTIME_CLK_SEC]
+ seqcnt_check fail=1b
+
+ get_nsec_per_sec res=x9
+ lsl x9, x9, x12
+
+ get_clock_shifted_nsec res=x15, cycle_last=x10, mult=x11
+ get_ts_realtime res_sec=x10, res_nsec=x11, \
+ clock_nsec=x15, xtime_sec=x13, xtime_nsec=x14, nsec_to_sec=x9
/* Convert ns to us. */
mov x13, #1000
@@ -76,95 +158,102 @@ ENTRY(__kernel_gettimeofday)
stp w4, w5, [x1, #TZ_MINWEST]
3:
mov x0, xzr
- ret x2
+ ret
4:
/* Syscall fallback. */
mov x8, #__NR_gettimeofday
svc #0
- ret x2
+ ret
.cfi_endproc
ENDPROC(__kernel_gettimeofday)
+#define JUMPSLOT_MAX CLOCK_MONOTONIC_COARSE
+
/* int __kernel_clock_gettime(clockid_t clock_id, struct timespec *tp); */
ENTRY(__kernel_clock_gettime)
.cfi_startproc
- cmp w0, #CLOCK_REALTIME
- ccmp w0, #CLOCK_MONOTONIC, #0x4, ne
- b.ne 2f
-
- mov x2, x30
- .cfi_register x30, x2
-
- /* Get kernel timespec. */
+ cmp w0, #JUMPSLOT_MAX
+ b.hi syscall
adr vdso_data, _vdso_data
-1: seqcnt_acquire
- cbnz use_syscall, 7f
+ adr x_tmp, jumptable
+ add x_tmp, x_tmp, w0, uxtw #2
+ br x_tmp
+
+ ALIGN
+jumptable:
+ jump_slot jumptable, CLOCK_REALTIME, realtime
+ jump_slot jumptable, CLOCK_MONOTONIC, monotonic
+ b syscall
+ b syscall
+ b syscall
+ jump_slot jumptable, CLOCK_REALTIME_COARSE, realtime_coarse
+ jump_slot jumptable, CLOCK_MONOTONIC_COARSE, monotonic_coarse
+
+ .if (. - jumptable) != 4 * (JUMPSLOT_MAX + 1)
+ .error "Wrong jumptable size"
+ .endif
+
+ ALIGN
+realtime:
+ seqcnt_acquire
+ syscall_check fail=syscall
+ ldr x10, [vdso_data, #VDSO_CS_CYCLE_LAST]
+ ldp w11, w12, [vdso_data, #VDSO_CS_MULT]
+ ldp x13, x14, [vdso_data, #VDSO_XTIME_CLK_SEC]
+ seqcnt_check fail=realtime
- bl __do_get_tspec
- seqcnt_check w9, 1b
+ /* All computations are done with left-shifted nsecs. */
+ get_nsec_per_sec res=x9
+ lsl x9, x9, x12
- mov x30, x2
+ get_clock_shifted_nsec res=x15, cycle_last=x10, mult=x11
+ get_ts_realtime res_sec=x10, res_nsec=x11, \
+ clock_nsec=x15, xtime_sec=x13, xtime_nsec=x14, nsec_to_sec=x9
+ clock_gettime_return, shift=1
- cmp w0, #CLOCK_MONOTONIC
- b.ne 6f
+ ALIGN
+monotonic:
+ seqcnt_acquire
+ syscall_check fail=syscall
+ ldr x10, [vdso_data, #VDSO_CS_CYCLE_LAST]
+ ldp w11, w12, [vdso_data, #VDSO_CS_MULT]
+ ldp x13, x14, [vdso_data, #VDSO_XTIME_CLK_SEC]
+ ldp x3, x4, [vdso_data, #VDSO_WTM_CLK_SEC]
+ seqcnt_check fail=monotonic
- /* Get wtm timespec. */
- ldp x13, x14, [vdso_data, #VDSO_WTM_CLK_SEC]
+ /* All computations are done with left-shifted nsecs. */
+ lsl x4, x4, x12
+ get_nsec_per_sec res=x9
+ lsl x9, x9, x12
- /* Check the sequence counter. */
- seqcnt_read w9
- seqcnt_check w9, 1b
- b 4f
-2:
- cmp w0, #CLOCK_REALTIME_COARSE
- ccmp w0, #CLOCK_MONOTONIC_COARSE, #0x4, ne
- b.ne 8f
+ get_clock_shifted_nsec res=x15, cycle_last=x10, mult=x11
+ get_ts_realtime res_sec=x10, res_nsec=x11, \
+ clock_nsec=x15, xtime_sec=x13, xtime_nsec=x14, nsec_to_sec=x9
- /* xtime_coarse_nsec is already right-shifted */
- mov x12, #0
+ add_ts sec=x10, nsec=x11, ts_sec=x3, ts_nsec=x4, nsec_to_sec=x9
+ clock_gettime_return, shift=1
- /* Get coarse timespec. */
- adr vdso_data, _vdso_data
-3: seqcnt_acquire
+ ALIGN
+realtime_coarse:
+ seqcnt_acquire
ldp x10, x11, [vdso_data, #VDSO_XTIME_CRS_SEC]
+ seqcnt_check fail=realtime_coarse
+ clock_gettime_return
- /* Get wtm timespec. */
+ ALIGN
+monotonic_coarse:
+ seqcnt_acquire
+ ldp x10, x11, [vdso_data, #VDSO_XTIME_CRS_SEC]
ldp x13, x14, [vdso_data, #VDSO_WTM_CLK_SEC]
+ seqcnt_check fail=monotonic_coarse
- /* Check the sequence counter. */
- seqcnt_read w9
- seqcnt_check w9, 3b
-
- cmp w0, #CLOCK_MONOTONIC_COARSE
- b.ne 6f
-4:
- /* Add on wtm timespec. */
- add x10, x10, x13
- lsl x14, x14, x12
- add x11, x11, x14
+ /* Computations are done in (non-shifted) nsecs. */
+ get_nsec_per_sec res=x9
+ add_ts sec=x10, nsec=x11, ts_sec=x13, ts_nsec=x14, nsec_to_sec=x9
+ clock_gettime_return
- /* Normalise the new timespec. */
- mov x15, #NSEC_PER_SEC_LO16
- movk x15, #NSEC_PER_SEC_HI16, lsl #16
- lsl x15, x15, x12
- cmp x11, x15
- b.lt 5f
- sub x11, x11, x15
- add x10, x10, #1
-5:
- cmp x11, #0
- b.ge 6f
- add x11, x11, x15
- sub x10, x10, #1
-
-6: /* Store to the user timespec. */
- lsr x11, x11, x12
- stp x10, x11, [x1, #TSPEC_TV_SEC]
- mov x0, xzr
- ret
-7:
- mov x30, x2
-8: /* Syscall fallback. */
+ ALIGN
+syscall: /* Syscall fallback. */
mov x8, #__NR_clock_gettime
svc #0
ret
@@ -203,46 +292,3 @@ ENTRY(__kernel_clock_getres)
.quad CLOCK_COARSE_RES
.cfi_endproc
ENDPROC(__kernel_clock_getres)
-
-/*
- * Read the current time from the architected counter.
- * Expects vdso_data to be initialised.
- * Clobbers the temporary registers (x9 - x15).
- * Returns:
- * - w9 = vDSO sequence counter
- * - (x10, x11) = (ts->tv_sec, shifted ts->tv_nsec)
- * - w12 = cs_shift
- */
-ENTRY(__do_get_tspec)
- .cfi_startproc
-
- /* Read from the vDSO data page. */
- ldr x10, [vdso_data, #VDSO_CS_CYCLE_LAST]
- ldp x13, x14, [vdso_data, #VDSO_XTIME_CLK_SEC]
- ldp w11, w12, [vdso_data, #VDSO_CS_MULT]
- seqcnt_read w9
-
- /* Read the virtual counter. */
- isb
- mrs x15, cntvct_el0
-
- /* Calculate cycle delta and convert to ns. */
- sub x10, x15, x10
- /* We can only guarantee 56 bits of precision. */
- movn x15, #0xff00, lsl #48
- and x10, x15, x10
- mul x10, x10, x11
-
- /* Use the kernel time to calculate the new timespec. */
- mov x11, #NSEC_PER_SEC_LO16
- movk x11, #NSEC_PER_SEC_HI16, lsl #16
- lsl x11, x11, x12
- add x15, x10, x14
- udiv x14, x15, x11
- add x10, x13, x14
- mul x13, x14, x11
- sub x11, x15, x13
-
- ret
- .cfi_endproc
-ENDPROC(__do_get_tspec)
--
2.9.0
More information about the linux-arm-kernel
mailing list