linux-kernel - Re: [PATCH] x86/kvmclock: set offset for kvm unstable clock

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Date:   Mon, 28 Jan 2019 10:15:53 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>, rkrcmar@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        nuxi@...lt24.org, carnil@...ian.org, t.glaser@...ent.de,
        asmadeus@...ewreck.org
Subject: Re: [PATCH] x86/kvmclock: set offset for kvm unstable clock

On 26/01/19 18:49, Pavel Tatashin wrote:
> VMs may show incorrect uptime and dmesg printk offsets on hypervisors with
> unstable clock. The problem is produced when VM is rebooted without exiting
> from qemu.
> 
> The fix is to calculate clock offset not only for stable clock but for
> unstable clock as well, and use kvm_sched_clock_read() which substracts
> the offset for both clocks.
> 
> This is safe, because pvclock_clocksource_read() does the right thing and
> makes sure that clock always goes forward, so once offset is calculated
> with unstable clock, we won't get new reads that are smaller than offset,
> and thus won't get negative results.
> 
> Thank you Jon DeVree for helping to reproduce this issue.
> 
> Fixes: 857baa87b642 ("sched/clock: Enable sched clock early")
> 
> Reported-by: Dominique Martinet <asmadeus@...ewreck.org>
> Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
> ---
>  arch/x86/kernel/kvmclock.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index e811d4d1c824..d908a37bf3f3 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -104,12 +104,8 @@ static u64 kvm_sched_clock_read(void)
>  
>  static inline void kvm_sched_clock_init(bool stable)
>  {
> -	if (!stable) {
> -		pv_ops.time.sched_clock = kvm_clock_read;
> +	if (!stable)
>  		clear_sched_clock_stable();
> -		return;
> -	}
> -
>  	kvm_sched_clock_offset = kvm_clock_read();
>  	pv_ops.time.sched_clock = kvm_sched_clock_read;
>  
> 

Queued, thanks.

Pa