diff options
author | Peter Zijlstra <peterz@infradead.org> | 2023-05-19 13:21:06 +0300 |
---|---|---|
committer | Peter Zijlstra <peterz@infradead.org> | 2023-06-05 22:11:07 +0300 |
commit | 77750f78b0b3247c64b9821b49158cafe0506880 (patch) | |
tree | 0b3efda22d56270b3e3b34a072d0865f1ddc2249 /arch/x86/include/asm/vdso | |
parent | fc4a0db4149afcdae2527f0d8c376accca34adc9 (diff) | |
download | linux-77750f78b0b3247c64b9821b49158cafe0506880.tar.xz |
x86/vdso: Fix gettimeofday masking
Because of how the virtual clocks use U64_MAX as an exception value
instead of a valid time, the clocks can no longer be assumed to wrap
cleanly. This is then compounded by arch_vdso_cycles_ok() rejecting
everything with the MSB/Sign-bit set.
Therefore, the effective mask becomes S64_MAX, and the comment with
vdso_calc_delta() that states the mask is U64_MAX and isn't optimized
out is just plain silly.
Now, the code has a negative filter -- to deal with TSC wobbles:
if (cycles > last)
which is just plain wrong, because it should've been written as:
if ((s64)(cycles - last) > 0)
to take wrapping into account, but per all the above, we don't
actually wrap on u64 anymore.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mikelley@microsoft.com> # Hyper-V
Link: https://lore.kernel.org/r/20230519102715.704767397@infradead.org
Diffstat (limited to 'arch/x86/include/asm/vdso')
-rw-r--r-- | arch/x86/include/asm/vdso/gettimeofday.h | 39 |
1 files changed, 28 insertions, 11 deletions
diff --git a/arch/x86/include/asm/vdso/gettimeofday.h b/arch/x86/include/asm/vdso/gettimeofday.h index 4cf6794f9d68..0badf0a9f03d 100644 --- a/arch/x86/include/asm/vdso/gettimeofday.h +++ b/arch/x86/include/asm/vdso/gettimeofday.h @@ -231,14 +231,17 @@ static u64 vread_pvclock(void) ret = __pvclock_read_cycles(pvti, rdtsc_ordered()); } while (pvclock_read_retry(pvti, version)); - return ret; + return ret & S64_MAX; } #endif #ifdef CONFIG_HYPERV_TIMER static u64 vread_hvclock(void) { - return hv_read_tsc_page(&hvclock_page); + u64 ret = hv_read_tsc_page(&hvclock_page); + if (likely(ret != U64_MAX)) + ret &= S64_MAX; + return ret; } #endif @@ -246,7 +249,7 @@ static inline u64 __arch_get_hw_counter(s32 clock_mode, const struct vdso_data *vd) { if (likely(clock_mode == VDSO_CLOCKMODE_TSC)) - return (u64)rdtsc_ordered(); + return (u64)rdtsc_ordered() & S64_MAX; /* * For any memory-mapped vclock type, we need to make sure that gcc * doesn't cleverly hoist a load before the mode check. Otherwise we @@ -284,6 +287,9 @@ static inline bool arch_vdso_clocksource_ok(const struct vdso_data *vd) * which can be invalidated asynchronously and indicate invalidation by * returning U64_MAX, which can be effectively tested by checking for a * negative value after casting it to s64. + * + * This effectively forces a S64_MAX mask on the calculations, unlike the + * U64_MAX mask normally used by x86 clocksources. */ static inline bool arch_vdso_cycles_ok(u64 cycles) { @@ -303,18 +309,29 @@ static inline bool arch_vdso_cycles_ok(u64 cycles) * @last. If not then use @last, which is the base time of the current * conversion period. * - * This variant also removes the masking of the subtraction because the - * clocksource mask of all VDSO capable clocksources on x86 is U64_MAX - * which would result in a pointless operation. The compiler cannot - * optimize it away as the mask comes from the vdso data and is not compile - * time constant. + * This variant also uses a custom mask because while the clocksource mask of + * all the VDSO capable clocksources on x86 is U64_MAX, the above code uses + * U64_MASK as an exception value, additionally arch_vdso_cycles_ok() above + * declares everything with the MSB/Sign-bit set as invalid. Therefore the + * effective mask is S64_MAX. */ static __always_inline u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult) { - if (cycles > last) - return (cycles - last) * mult; - return 0; + /* + * Due to the MSB/Sign-bit being used as invald marker (see + * arch_vdso_cycles_valid() above), the effective mask is S64_MAX. + */ + u64 delta = (cycles - last) & S64_MAX; + + /* + * Due to the above mentioned TSC wobbles, filter out negative motion. + * Per the above masking, the effective sign bit is now bit 62. + */ + if (unlikely(delta & (1ULL << 62))) + return 0; + + return delta * mult; } #define vdso_calc_delta vdso_calc_delta |