Re: [Minios-devel] [UNIKRAFT PATCHv4 23/43] plat/kvm: Add Arm64 virtual timer library to provide ticks

[Sharan Santhanam <sharan.santhanam@xxxxxxxxx>]

Hello Wei Chen,


On 07/25/2018 08:41 AM, Wei Chen wrote:
> Hi Sharan,
>
> > -----Original Message-----
> > From: Sharan Santhanam <sharan.santhanam@xxxxxxxxx>
> > Sent: 2018年7月24日 20:03
> > To: minios-devel@xxxxxxxxxxxxx
> > Cc: Wei Chen <Wei.Chen@xxxxxxx>; Simon Kuenzer <simon.kuenzer@xxxxxxxxx>
> > Subject: Re: [Minios-devel] [UNIKRAFT PATCHv4 23/43] plat/kvm: Add Arm64
> > virtual timer library to provide ticks
> >
> > Hello Wei Chen,
> >
> > Please find my comment inline:
> >
> > On 07/06/2018 11:03 AM, Wei Chen wrote:
> > > On KVM platform, print debug message will use ukplat_monotonic_clock
> > > to provide timestamp. So we implement this simple virtual timer
> > > library for timestamp.
> > >
> > > Signed-off-by: Wei Chen <Wei.Chen@xxxxxxx>
> > > ---
> > >    plat/kvm/arm/time.c | 127 ++++++++++++++++++++++++++++++++++++++++++++
> > >    1 file changed, 127 insertions(+)
> > >    create mode 100644 plat/kvm/arm/time.c
> > >
> > > diff --git a/plat/kvm/arm/time.c b/plat/kvm/arm/time.c
> > > new file mode 100644
> > > index 0000000..ab4968f
> > > --- /dev/null
> > > +++ b/plat/kvm/arm/time.c
> > > @@ -0,0 +1,127 @@
> > > +/* SPDX-License-Identifier: BSD-3-Clause */
> > > +/*
> > > + * Authors: Wei Chen <Wei.Chen@xxxxxxx>
> > > + *
> > > + * Copyright (c) 2018, Arm Ltd. All rights reserved.
> > > + *
> > > + * Redistribution and use in source and binary forms, with or without
> > > + * modification, are permitted provided that the following conditions
> > > + * are met:
> > > + *
> > > + * 1. Redistributions of source code must retain the above copyright
> > > + *    notice, this list of conditions and the following disclaimer.
> > > + * 2. Redistributions in binary form must reproduce the above copyright
> > > + *    notice, this list of conditions and the following disclaimer in the
> > > + *    documentation and/or other materials provided with the distribution.
> > > + * 3. Neither the name of the copyright holder nor the names of its
> > > + *    contributors may be used to endorse or promote products derived from
> > > + *    this software without specific prior written permission.
> > > + *
> > > + * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
> > IS"
> > > + * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
> > THE
> > > + * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
> > PURPOSE
> > > + * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS
> > BE
> > > + * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
> > > + * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
> > > + * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
> > > + * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
> > > + * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
> > > + * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
> > THE
> > > + * POSSIBILITY OF SUCH DAMAGE.
> > > + *
> > > + * THIS HEADER MAY NOT BE EXTRACTED OR MODIFIED IN ANY WAY.
> > > + */
> > > +#include <stdlib.h>
> > > +#include <uk/assert.h>
> > > +#include <uk/plat/time.h>
> > > +
> > > +static uint64_t cntvct_at_init;
> > > +static uint32_t counter_freq;
> > > +/*
> > > + * Shift factor for TSC scaling multiplier; referred to as S in the
> > following
> > > + * comments.
> > > + */
> > > +static uint8_t tsc_shift;
> > > +
> > > +/* Multiplier for converting TSC ticks to nsecs. (0.S) fixed point. */
> > > +static uint32_t ns_per_tick;
> > > +
> > > +/*
> > > + * System Time
> > > + * 64 bit value containing the nanoseconds elapsed since boot time.
> > > + * This value is adjusted by frequency drift.
> > > + * NOW() returns the current time.
> > > + * The other macros are for convenience to approximate short intervals
> > > + * of real time into system time
> > > + */
> > > +#define NSEC_PER_SEC 1000000000ULL
> > > +
> > > +static inline uint64_t ticks_to_ns(uint64_t ticks)
> > > +{
> > > +return (ns_per_tick * ticks) >> tsc_shift;
> > > +}
> > > +
> > > +static inline uint64_t get_counter_frequency(void)
> > > +{
> > > +uint64_t frq;
> > > +
> > > +__asm__ __volatile__("mrs %0, cntfrq_el0" : "=r" (frq):: "memory");
> > > +
> > > +return frq;
> > > +}
> > > +
> > > +static inline uint64_t read_virtual_count(void)
> > > +{
> > > +uint64_t val;
> > > +
> > Cortex-A73 counter returns a wrong value if read while crossing a 32bit
> > boundary
> >
> > Please refer to errata #858921, in document[1]. For instance, Linux[2]
> > work around the problem by using
> >
> >           do {
> >                   _old = read_sysreg(reg);
> >                   _new = read_sysreg(reg);
> >                   _retries--;
> >           } while (unlikely((_new - _old) >> 5) && _retries);
> Thanks for giving so detailed documents : )
> While I was doing Arm64 porting work, I had considered whether or not to
> support errata. Because we don't have an errata framework. For this
> specific errata, we can support it easily by providing an option and
> #ifdef/else for users to enable or disable. But if we want to avoid
> users to enable this errata workaround for CPUs without this errata,
> we have to check the CPU IDs and features further.
>
> I can implement this errata workaround for counter in next version,
> but we'd better have a framework : )
For the next version of this patch, I think we should provide a fix for 
the timer "using ifdef" and introduces a new patch series for the errata 
framework.

> > > +__asm__ __volatile__("mrs %0, cntvct_el0" : "=r" (val)::);
> > > +return val;
> > > +}
> > > +
> > > +/* monotonic_clock(): returns # of nanoseconds passed since time_init()
> > > + * Note: This function is required to return accurate
> > > + *       time even in the absence of multiple timer ticks.
> > > + */
> > > +__nsec ukplat_monotonic_clock(void)
> > > +{
> > > +return (__nsec) ticks_to_ns(read_virtual_count() - cntvct_at_init);
> > > +}
> > > +
> > > +void ukplat_time_init(void)
> > > +{
> > > +/*
> > > + * Calculate TSC shift factor and scaling multiplier.
> > > + *
> > > + * tsc_shift (S) needs to be the largest (<=32) shift factor where the
> > > + * result of the tsc_mult calculcation below fits into uint32_t without
> > > + * truncation. Note that we disallow an S of zero to ensure the loop
> > always
> > > + * terminates.
> > > + *
> > > + * (0.S) tsc_mult = NSEC_PER_SEC (S.S) / tsc_freq (S.0)
> > > + */
> > > +uint64_t tmp;
> > > +
> > > +counter_freq = get_counter_frequency();
> > > +tsc_shift = 32;
> > > +do {
> > > +tmp = (NSEC_PER_SEC << tsc_shift) / counter_freq;
> > > +if ((tmp & 0xFFFFFFFF00000000L) == 0L)
> > > +ns_per_tick = (uint32_t)tmp;
> > > +else
> > > +tsc_shift--;
> > > +} while (tsc_shift > 0 && ns_per_tick == 0L);
> > > +#if 0 /* assert has been implemented for kvm */
> > > +assert(ns_per_tick != 0L);
> > > +#endif
> > > +
> > > +/*
> > > + * Monotonic time begins at tsc_base (first read of TSC before
> > > + * calibration).
> > > + */
> > > +cntvct_at_init = read_virtual_count();
> > > +}
> > > +
> > > +void ukplat_time_fini(void)
> > > +{
> > > +/* TODO */
> > > +}
> >
> > [1]https://elixir.bootlin.com/linux/latest/source/Documentation/arm64/silicon-
> > errata.txt
> > [2] https://lkml.org/lkml/2017/4/7/369
> >
> >
> > Thanks & Regards
> > Sharan
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Thanks & Regards
Sharan

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