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Commit 3e89bf35 authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

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: default avatarThomas Gleixner <tglx@linutronix.de>
Acked-by: default avatarAndy 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
parent 4f72adc5

arch/x86/entry/vdso/vclock_gettime.c

+7 −32
Original line number Diff line number Diff line
@@ -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;

}

#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)));