x86/vdso: Move cycle_last handling into the caller

Dereferencing gtod->cycle_last all over the place and foing the cycles <
last comparison in the vclock read functions generates horrible code. Doing
it at the call site is much better and gains a few cycles both for TSC and
pvclock.

Caveat: This adds the comparison to the hyperv vclock as well, but I have
no way to test that.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Matt Rickard <matt@softrans.com.au>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: devel@linuxdriverproject.org
Cc: virtualization@lists.linux-foundation.org
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Juergen Gross <jgross@suse.com>
Link: https://lkml.kernel.org/r/20180917130707.741440803@linutronix.de

1 files changed, 7 insertions, 32 deletions

diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index 40105024a210..b7ccbff26a3f 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -77,9 +77,8 @@ static notrace const struct pvclock_vsyscall_time_info *get_pvti0(void)
 static notrace u64 vread_pvclock(void)
 {
         const struct pvclock_vcpu_time_info *pvti = &get_pvti0()->pvti;
-        u64 ret;
-        u64 last;
         u32 version;
+        u64 ret;
 
         /*
          * Note: The kernel and hypervisor must guarantee that cpu ID
@@ -112,13 +111,7 @@ static notrace u64 vread_pvclock(void)
                 ret = __pvclock_read_cycles(pvti, rdtsc_ordered());
         } while (pvclock_read_retry(pvti, version));
 
-        /* refer to vread_tsc() comment for rationale */
-        last = gtod->cycle_last;
-
-        if (likely(ret >= last))
-                return ret;
-
-        return last;
+        return ret;
 }
 #endif
 #ifdef CONFIG_HYPERV_TSCPAGE
@@ -131,30 +124,10 @@ static notrace u64 vread_hvclock(void)
 }
 #endif
 
-notrace static u64 vread_tsc(void)
-{
-        u64 ret = (u64)rdtsc_ordered();
-        u64 last = gtod->cycle_last;
-
-        if (likely(ret >= last))
-                return ret;
-
-        /*
-         * GCC likes to generate cmov here, but this branch is extremely
-         * predictable (it's just a function of time and the likely is
-         * very likely) and there's a data dependence, so force GCC
-         * to generate a branch instead.  I don't barrier() because
-         * we don't actually need a barrier, and if this function
-         * ever gets inlined it will generate worse code.
-         */
-        asm volatile ("");
-        return last;
-}
-
 notrace static inline u64 vgetcyc(int mode)
 {
         if (mode == VCLOCK_TSC)
-                return vread_tsc();
+                return (u64)rdtsc_ordered();
 #ifdef CONFIG_PARAVIRT_CLOCK
         else if (mode == VCLOCK_PVCLOCK)
                 return vread_pvclock();
@@ -169,17 +142,19 @@ notrace static inline u64 vgetcyc(int mode)
 notrace static int do_hres(clockid_t clk, struct timespec *ts)
 {
         struct vgtod_ts *base = &gtod->basetime[clk];
+        u64 cycles, last, ns;
         unsigned int seq;
-        u64 cycles, ns;
 
         do {
                 seq = gtod_read_begin(gtod);
                 ts->tv_sec = base->sec;
                 ns = base->nsec;
+                last = gtod->cycle_last;
                 cycles = vgetcyc(gtod->vclock_mode);
                 if (unlikely((s64)cycles < 0))
                         return vdso_fallback_gettime(clk, ts);
-                ns += (cycles - gtod->cycle_last) * gtod->mult;
+                if (cycles > last)
+                        ns += (cycles - last) * gtod->mult;
                 ns >>= gtod->shift;
         } while (unlikely(gtod_read_retry(gtod, seq)));