linux-kernel - Re: [PATCH 1/2] KVM: x86: reorganize pvclock_gtod

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Message-ID: <87tv4mqug3.fsf@vitty.brq.redhat.com>
Date:   Thu, 23 Jan 2020 12:32:44 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     mtosatti@...hat.com, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: x86: reorganize pvclock_gtod_data members

Paolo Bonzini <pbonzini@...hat.com> writes:

> We will need a copy of tk->offs_boot in the next patch.  Store it and
> cleanup the struct: instead of storing tk->tkr_xxx.base with the tk->offs_boot
> included, store the raw value in struct pvclock_clock and sum tk->offs_boot
> in do_monotonic_raw and do_realtime.   tk->tkr_xxx.xtime_nsec also moves
> to struct pvclock_clock.
>
> While at it, fix a (usually harmless) typo in do_monotonic_raw, which
> was using gtod->clock.shift instead of gtod->raw_clock.shift.
>
> Fixes: 53fafdbb8b21f ("KVM: x86: switch KVMCLOCK base to monotonic raw clock")
> Cc: stable@...r.kernel.org
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 29 ++++++++++++-----------------
>  1 file changed, 12 insertions(+), 17 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 89621025577a..1b4273cce63c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1532,6 +1532,8 @@ struct pvclock_clock {
>  	u64 mask;
>  	u32 mult;
>  	u32 shift;
> +	u64 base_cycles;
> +	u64 offset;
>  };
>  
>  struct pvclock_gtod_data {
> @@ -1540,11 +1542,8 @@ struct pvclock_gtod_data {
>  	struct pvclock_clock clock; /* extract of a clocksource struct */
>  	struct pvclock_clock raw_clock; /* extract of a clocksource struct */
>  
> -	u64		boot_ns_raw;
> -	u64		boot_ns;
> -	u64		nsec_base;
> +	ktime_t		offs_boot;
>  	u64		wall_time_sec;
> -	u64		monotonic_raw_nsec;
>  };
>  
>  static struct pvclock_gtod_data pvclock_gtod_data;
> @@ -1552,10 +1551,6 @@ struct pvclock_gtod_data {
>  static void update_pvclock_gtod(struct timekeeper *tk)
>  {
>  	struct pvclock_gtod_data *vdata = &pvclock_gtod_data;
> -	u64 boot_ns, boot_ns_raw;
> -
> -	boot_ns = ktime_to_ns(ktime_add(tk->tkr_mono.base, tk->offs_boot));
> -	boot_ns_raw = ktime_to_ns(ktime_add(tk->tkr_raw.base, tk->offs_boot));
>  
>  	write_seqcount_begin(&vdata->seq);
>  
> @@ -1565,20 +1560,20 @@ static void update_pvclock_gtod(struct timekeeper *tk)
>  	vdata->clock.mask		= tk->tkr_mono.mask;
>  	vdata->clock.mult		= tk->tkr_mono.mult;
>  	vdata->clock.shift		= tk->tkr_mono.shift;
> +	vdata->clock.base_cycles	= tk->tkr_mono.xtime_nsec;
> +	vdata->clock.offset		= tk->tkr_mono.base;
>  
>  	vdata->raw_clock.vclock_mode	= tk->tkr_raw.clock->archdata.vclock_mode;
>  	vdata->raw_clock.cycle_last	= tk->tkr_raw.cycle_last;
>  	vdata->raw_clock.mask		= tk->tkr_raw.mask;
>  	vdata->raw_clock.mult		= tk->tkr_raw.mult;
>  	vdata->raw_clock.shift		= tk->tkr_raw.shift;
> -
> -	vdata->boot_ns			= boot_ns;
> -	vdata->nsec_base		= tk->tkr_mono.xtime_nsec;
> +	vdata->raw_clock.base_cycles	= tk->tkr_raw.xtime_nsec;
> +	vdata->raw_clock.offset		= tk->tkr_raw.base;

Likely a personal preference but the suggested naming is a bit
confusing: we use 'base_cycles' to keep 'xtime_nsec' and 'offset' to
keep ... 'base'. Not that I think that 'struct timekeeper' is perfect
but at least it is documented. Should we maybe just stick to it (and
name 'struct pvclock_clock' fields accordingly?)

>  
>  	vdata->wall_time_sec            = tk->xtime_sec;
>  
> -	vdata->boot_ns_raw		= boot_ns_raw;
> -	vdata->monotonic_raw_nsec	= tk->tkr_raw.xtime_nsec;
> +	vdata->offs_boot		= tk->offs_boot;
>  
>  	write_seqcount_end(&vdata->seq);
>  }
> @@ -2048,10 +2043,10 @@ static int do_monotonic_raw(s64 *t, u64 *tsc_timestamp)
>  
>  	do {
>  		seq = read_seqcount_begin(&gtod->seq);
> -		ns = gtod->monotonic_raw_nsec;
> +		ns = gtod->raw_clock.base_cycles;
>  		ns += vgettsc(&gtod->raw_clock, tsc_timestamp, &mode);
> -		ns >>= gtod->clock.shift;
> -		ns += gtod->boot_ns_raw;
> +		ns >>= gtod->raw_clock.shift;
> +		ns += ktime_to_ns(ktime_add(gtod->raw_clock.offset, gtod->offs_boot));
>  	} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
>  	*t = ns;
>  
> @@ -2068,7 +2063,7 @@ static int do_realtime(struct timespec64 *ts, u64 *tsc_timestamp)
>  	do {
>  		seq = read_seqcount_begin(&gtod->seq);
>  		ts->tv_sec = gtod->wall_time_sec;
> -		ns = gtod->nsec_base;
> +		ns = gtod->clock.base_cycles;
>  		ns += vgettsc(&gtod->clock, tsc_timestamp, &mode);
>  		ns >>= gtod->clock.shift;
>  	} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));

FWIW,

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly