linux-kernel - Re: [patch 0/3] vdso: Unbreak VDSO with PV and HyperV clocksources

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Message-ID: <CAJfpegtgQZvjjapsgQsG6AGGmNsHoGaczVc9=nw941a4-atmGw@mail.gmail.com>
Date:   Tue, 9 Jun 2020 15:10:15 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Christophe Leroy <christophe.leroy@....fr>
Subject: Re: [patch 0/3] vdso: Unbreak VDSO with PV and HyperV clocksources

On Sun, Jun 7, 2020 at 11:36 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Miklos reported [1] that the recent VDSO changes broke paravirt clocksource
> based VDSO in the case that the clocksource is invalidated by the
> hypervisor which happens after a suspend/resume cycle of the host.
>
> The result is a stale clocksource which is about 2200 seconds ahead of the
> actual time and jumps forward by 2200 seconds once 2200 seconds have
> elapsed.
>
> The reason for this is the core code change which optimized the VDSO
> clocksource validation by checking for the clocksource mode instead of
> using the rather subtle check for the clocksource read return value whether
> it has bit 63 set.
>
> For some reason my brain blanked when doing that change, even if I should
> have known better.
>
> The following series restores the previous behaviour but preserves the
> initially intended optimization for architectures which don't need that PV
> handling.

Thanks for fixing.

Tested-by: Miklos Szeredi <mszeredi@...hat.com>

Miklos