linux-kernel - Re: [PATCH] kvm: x86: fix reversed timespec values in PV wall clock

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Message-ID: <CAOQ_Qsh2aXWgNrSixcV4vEOr0NtPTDtZY1h8EWUQr0vba61BxQ@mail.gmail.com>
Date:   Fri, 24 Jul 2020 11:45:23 -0700
From:   Oliver Upton <oupton@...gle.com>
To:     kvm list <kvm@...r.kernel.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Peter Shier <pshier@...gle.com>
Subject: Re: [PATCH] kvm: x86: fix reversed timespec values in PV wall clock

On Fri, Jul 24, 2020 at 10:35 AM Oliver Upton <oupton@...gle.com> wrote:
>
> commit 8171cd68806b ("KVM: x86: use raw clock values consistently")
> causes KVM to accidentally write seconds to the nanoseconds field (and
> vice versa) in the KVM wall clock. Fix it by reversing this accidental
> switch. Modulo the written nanoseconds value by NSEC_PER_SEC to correct
> for the amount of time represented as seconds.
>
> Fixes: 8171cd68806b ("KVM: x86: use raw clock values consistently")
> Cc: stable@...r.kernel.org
> Reviewed-by: Jim Mattson <jmattson@...gle.com>
> Reviewed-by: Peter Shier <pshier@...gle.com>
> Signed-off-by: Oliver Upton <oupton@...gle.com>
> ---
>  Parent commit: c34b26b98cac ("KVM: MIPS: clean up redundant 'kvm_run' parameters")
>
>  arch/x86/kvm/x86.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e27d3db7e43f..86228cc6b29e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1809,8 +1809,9 @@ static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock)
>          */
>         wall_nsec = ktime_get_real_ns() - get_kvmclock_ns(kvm);
>
> -       wc.nsec = do_div(wall_nsec, 1000000000);
> -       wc.sec = (u32)wall_nsec; /* overflow in 2106 guest time */
> +       /* overflow in 2106 guest time */
> +       wc.sec = (u32)do_div(wall_nsec, NSEC_PER_SEC);
> +       wc.nsec = wall_nsec % NSEC_PER_SEC;
>         wc.version = version;
>
>         kvm_write_guest(kvm, wall_clock, &wc, sizeof(wc));
> --
> 2.28.0.rc0.142.g3c755180ce-goog
>

Disregard, I crossed wires in my reading of do_div().