powerpc: Rework VDSO gettimeofday to prevent time going backwards

Currently it is possible for userspace to see the result of
gettimeofday() going backwards by 1 microsecond, assuming that
userspace is using the gettimeofday() in the VDSO.  The VDSO
gettimeofday() algorithm computes the time in "xsecs", which are
units of 2^-20 seconds, or approximately 0.954 microseconds,
using the algorithm

	now = (timebase - tb_orig_stamp) * tb_to_xs + stamp_xsec

and then converts the time in xsecs to seconds and microseconds.

The kernel updates the tb_orig_stamp and stamp_xsec values every
tick in update_vsyscall().  If the length of the tick is not an
integer number of xsecs, then some precision is lost in converting
the current time to xsecs.  For example, with CONFIG_HZ=1000, the
tick is 1ms long, which is 1048.576 xsecs.  That means that
stamp_xsec will advance by either 1048 or 1049 on each tick.
With the right conditions, it is possible for userspace to get
(timebase - tb_orig_stamp) * tb_to_xs being 1049 if the kernel is
slightly late in updating the vdso_datapage, and then for stamp_xsec
to advance by 1048 when the kernel does update it, and for userspace
to then see (timebase - tb_orig_stamp) * tb_to_xs being zero due to
integer truncation.  The result is that time appears to go backwards
by 1 microsecond.

To fix this we change the VDSO gettimeofday to use a new field in the
VDSO datapage which stores the nanoseconds part of the time as a
fractional number of seconds in a 0.32 binary fraction format.
(Or put another way, as a 32-bit number in units of 0.23283 ns.)
This is convenient because we can use the mulhwu instruction to
convert it to either microseconds or nanoseconds.

Since it turns out that computing the time of day using this new field
is simpler than either using stamp_xsec (as gettimeofday does) or
stamp_xtime.tv_nsec (as clock_gettime does), this converts both
gettimeofday and clock_gettime to use the new field.  The existing
__do_get_tspec function is converted to use the new field and take
a parameter in r7 that indicates the desired resolution, 1,000,000
for microseconds or 1,000,000,000 for nanoseconds.  The __do_get_xsec
function is then unused and is deleted.

The new algorithm is

	now = ((timebase - tb_orig_stamp) << 12) * tb_to_xs
		+ (stamp_xtime_seconds << 32) + stamp_sec_fraction

with 'now' in units of 2^-32 seconds.  That is then converted to
seconds and either microseconds or nanoseconds with

	seconds = now >> 32
	partseconds = ((now & 0xffffffff) * resolution) >> 32

The 32-bit VDSO code also makes a further simplification: it ignores
the bottom 32 bits of the tb_to_xs value, which is a 0.64 format binary
fraction.  Doing so gets rid of 4 multiply instructions.  Assuming
a timebase frequency of 1GHz or less and an update interval of no
more than 10ms, the upper 32 bits of tb_to_xs will be at least
4503599, so the error from ignoring the low 32 bits will be at most
2.2ns, which is more than an order of magnitude less than the time
taken to do gettimeofday or clock_gettime on our fastest processors,
so there is no possibility of seeing inconsistent values due to this.

This also moves update_gtod() down next to its only caller, and makes
update_vsyscall use the time passed in via the wall_time argument rather
than accessing xtime directly.  At present, wall_time always points to
xtime, but that could change in future.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 42 insertions, 142 deletions

diff --git a/arch/powerpc/kernel/vdso32/gettimeofday.S b/arch/powerpc/kernel/vdso32/gettimeofday.S
index ee038d4bf252..4ee09ee2e836 100644
--- a/arch/powerpc/kernel/vdso32/gettimeofday.S
+++ b/arch/powerpc/kernel/vdso32/gettimeofday.S
@@ -19,8 +19,10 @@
 /* Offset for the low 32-bit part of a field of long type */
 #ifdef CONFIG_PPC64
 #define LOPART  4
+#define TSPEC_TV_SEC    TSPC64_TV_SEC+LOPART
 #else
 #define LOPART  0
+#define TSPEC_TV_SEC    TSPC32_TV_SEC
 #endif
 
         .text
@@ -41,23 +43,11 @@ V_FUNCTION_BEGIN(__kernel_gettimeofday)
         mr      r9, r3                  /* datapage ptr in r9 */
         cmplwi  r10,0                   /* check if tv is NULL */
         beq     3f
-        bl      __do_get_xsec@local     /* get xsec from tb & kernel */
-        bne-    2f                      /* out of line -> do syscall */
-
-        /* seconds are xsec >> 20 */
-        rlwinm  r5,r4,12,20,31
-        rlwimi  r5,r3,12,0,19
-        stw     r5,TVAL32_TV_SEC(r10)
-
-        /* get remaining xsec and convert to usec. we scale
-         * up remaining xsec by 12 bits and get the top 32 bits
-         * of the multiplication
-         */
-        rlwinm  r5,r4,12,0,19
-        lis     r6,1000000@h
-        ori     r6,r6,1000000@l
-        mulhwu  r5,r5,r6
-        stw     r5,TVAL32_TV_USEC(r10)
+        lis     r7,1000000@ha           /* load up USEC_PER_SEC */
+        addi    r7,r7,1000000@l         /* so we get microseconds in r4 */
+        bl      __do_get_tspec@local    /* get sec/usec from tb & kernel */
+        stw     r3,TVAL32_TV_SEC(r10)
+        stw     r4,TVAL32_TV_USEC(r10)
 
 3:      cmplwi  r11,0                   /* check if tz is NULL */
         beq     1f
@@ -70,14 +60,6 @@ V_FUNCTION_BEGIN(__kernel_gettimeofday)
         crclr   cr0*4+so
         li      r3,0
         blr
-
-2:
-        mtlr    r12
-        mr      r3,r10
-        mr      r4,r11
-        li      r0,__NR_gettimeofday
-        sc
-        blr
   .cfi_endproc
 V_FUNCTION_END(__kernel_gettimeofday)
 
@@ -100,7 +82,8 @@ V_FUNCTION_BEGIN(__kernel_clock_gettime)
         mr      r11,r4                  /* r11 saves tp */
         bl      __get_datapage@local    /* get data page */
         mr      r9,r3                   /* datapage ptr in r9 */
-
+        lis     r7,NSEC_PER_SEC@h       /* want nanoseconds */
+        ori     r7,r7,NSEC_PER_SEC@l
 50:     bl      __do_get_tspec@local    /* get sec/nsec from tb & kernel */
         bne     cr1,80f                 /* not monotonic -> all done */
 
@@ -198,83 +181,12 @@ V_FUNCTION_END(__kernel_clock_getres)
 
 
 /*
- * This is the core of gettimeofday() & friends, it returns the xsec
- * value in r3 & r4 and expects the datapage ptr (non clobbered)
- * in r9. clobbers r0,r4,r5,r6,r7,r8.
- * When returning, r8 contains the counter value that can be reused
- * by the monotonic clock implementation
- */
-__do_get_xsec:
-  .cfi_startproc
-        /* Check for update count & load values. We use the low
-         * order 32 bits of the update count
-         */
-1:      lwz     r8,(CFG_TB_UPDATE_COUNT+LOPART)(r9)
-        andi.   r0,r8,1                 /* pending update ? loop */
-        bne-    1b
-        xor     r0,r8,r8                /* create dependency */
-        add     r9,r9,r0
-
-        /* Load orig stamp (offset to TB) */
-        lwz     r5,CFG_TB_ORIG_STAMP(r9)
-        lwz     r6,(CFG_TB_ORIG_STAMP+4)(r9)
-
-        /* Get a stable TB value */
-2:      mftbu   r3
-        mftbl   r4
-        mftbu   r0
-        cmpl    cr0,r3,r0
-        bne-    2b
-
-        /* Substract tb orig stamp. If the high part is non-zero, we jump to
-         * the slow path which call the syscall.
-         * If it's ok, then we have our 32 bits tb_ticks value in r7
-         */
-        subfc   r7,r6,r4
-        subfe.  r0,r5,r3
-        bne-    3f
-
-        /* Load scale factor & do multiplication */
-        lwz     r5,CFG_TB_TO_XS(r9)     /* load values */
-        lwz     r6,(CFG_TB_TO_XS+4)(r9)
-        mulhwu  r4,r7,r5
-        mulhwu  r6,r7,r6
-        mullw   r0,r7,r5
-        addc    r6,r6,r0
-
-        /* At this point, we have the scaled xsec value in r4 + XER:CA
-         * we load & add the stamp since epoch
-         */
-        lwz     r5,CFG_STAMP_XSEC(r9)
-        lwz     r6,(CFG_STAMP_XSEC+4)(r9)
-        adde    r4,r4,r6
-        addze   r3,r5
-
-        /* We now have our result in r3,r4. We create a fake dependency
-         * on that result and re-check the counter
-         */
-        or      r6,r4,r3
-        xor     r0,r6,r6
-        add     r9,r9,r0
-        lwz     r0,(CFG_TB_UPDATE_COUNT+LOPART)(r9)
-        cmpl    cr0,r8,r0               /* check if updated */
-        bne-    1b
-
-        /* Warning ! The caller expects CR:EQ to be set to indicate a
-         * successful calculation (so it won't fallback to the syscall
-         * method). We have overriden that CR bit in the counter check,
-         * but fortunately, the loop exit condition _is_ CR:EQ set, so
-         * we can exit safely here. If you change this code, be careful
-         * of that side effect.
-         */
-3:      blr
-  .cfi_endproc
-
-/*
- * This is the core of clock_gettime(), it returns the current
- * time in seconds and nanoseconds in r3 and r4.
+ * This is the core of clock_gettime() and gettimeofday(),
+ * it returns the current time in r3 (seconds) and r4.
+ * On entry, r7 gives the resolution of r4, either USEC_PER_SEC
+ * or NSEC_PER_SEC, giving r4 in microseconds or nanoseconds.
  * It expects the datapage ptr in r9 and doesn't clobber it.
- * It clobbers r0, r5, r6, r10 and returns NSEC_PER_SEC in r7.
+ * It clobbers r0, r5 and r6.
  * On return, r8 contains the counter value that can be reused.
  * This clobbers cr0 but not any other cr field.
  */
@@ -297,70 +209,58 @@ __do_get_tspec:
 2:      mftbu   r3
         mftbl   r4
         mftbu   r0
-        cmpl    cr0,r3,r0
+        cmplw   cr0,r3,r0
         bne-    2b
 
         /* Subtract tb orig stamp and shift left 12 bits.
          */
-        subfc   r7,r6,r4
+        subfc   r4,r6,r4
         subfe   r0,r5,r3
         slwi    r0,r0,12
-        rlwimi. r0,r7,12,20,31
-        slwi    r7,r7,12
+        rlwimi. r0,r4,12,20,31
+        slwi    r4,r4,12
 
-        /* Load scale factor & do multiplication */
+        /*
+         * Load scale factor & do multiplication.
+         * We only use the high 32 bits of the tb_to_xs value.
+         * Even with a 1GHz timebase clock, the high 32 bits of
+         * tb_to_xs will be at least 4 million, so the error from
+         * ignoring the low 32 bits will be no more than 0.25ppm.
+         * The error will just make the clock run very very slightly
+         * slow until the next time the kernel updates the VDSO data,
+         * at which point the clock will catch up to the kernel's value,
+         * so there is no long-term error accumulation.
+         */
         lwz     r5,CFG_TB_TO_XS(r9)     /* load values */
-        lwz     r6,(CFG_TB_TO_XS+4)(r9)
-        mulhwu  r3,r7,r6
-        mullw   r10,r7,r5
-        mulhwu  r4,r7,r5
-        addc    r10,r3,r10
+        mulhwu  r4,r4,r5
         li      r3,0
 
         beq+    4f                      /* skip high part computation if 0 */
         mulhwu  r3,r0,r5
-        mullw   r7,r0,r5
-        mulhwu  r5,r0,r6
-        mullw   r6,r0,r6
-        adde    r4,r4,r7
-        addze   r3,r3
+        mullw   r5,r0,r5
         addc    r4,r4,r5
         addze   r3,r3
-        addc    r10,r10,r6
-
-4:      addze   r4,r4                   /* add in carry */
-        lis     r7,NSEC_PER_SEC@h
-        ori     r7,r7,NSEC_PER_SEC@l
-        mulhwu  r4,r4,r7                /* convert to nanoseconds */
-
-        /* At this point, we have seconds & nanoseconds since the xtime
-         * stamp in r3+CA and r4.  Load & add the xtime stamp.
+4:
+        /* At this point, we have seconds since the xtime stamp
+         * as a 32.32 fixed-point number in r3 and r4.
+         * Load & add the xtime stamp.
          */
-#ifdef CONFIG_PPC64
-        lwz     r5,STAMP_XTIME+TSPC64_TV_SEC+LOPART(r9)
-        lwz     r6,STAMP_XTIME+TSPC64_TV_NSEC+LOPART(r9)
-#else
-        lwz     r5,STAMP_XTIME+TSPC32_TV_SEC(r9)
-        lwz     r6,STAMP_XTIME+TSPC32_TV_NSEC(r9)
-#endif
-        add     r4,r4,r6
+        lwz     r5,STAMP_XTIME+TSPEC_TV_SEC(r9)
+        lwz     r6,STAMP_SEC_FRAC(r9)
+        addc    r4,r4,r6
         adde    r3,r3,r5
 
-        /* We now have our result in r3,r4. We create a fake dependency
-         * on that result and re-check the counter
+        /* We create a fake dependency on the result in r3/r4
+         * and re-check the counter
          */
         or      r6,r4,r3
         xor     r0,r6,r6
         add     r9,r9,r0
         lwz     r0,(CFG_TB_UPDATE_COUNT+LOPART)(r9)
-        cmpl    cr0,r8,r0               /* check if updated */
+        cmplw   cr0,r8,r0               /* check if updated */
         bne-    1b
 
-        /* check for nanosecond overflow and adjust if necessary */
-        cmpw    r4,r7
-        bltlr                           /* all done if no overflow */
-        subf    r4,r7,r4                /* adjust if overflow */
-        addi    r3,r3,1
+        mulhwu  r4,r4,r7                /* convert to micro or nanoseconds */
 
         blr
   .cfi_endproc