[PATCH 1/1] riscv/kvm: Fix VM hang in case of timer delta being zero.

[Anup Patel]

On Fri, Feb 10, 2023 at 7:21 PM Rajnesh Kanwal <rkanwal at rivosinc.com> wrote:
>
> In case when VCPU is blocked due to WFI, we schedule the timer
> from `kvm_riscv_vcpu_timer_blocking()` to keep timer interrupt
> ticking.
>
> But in case when delta_ns comes to be zero, we never schedule
> the timer and VCPU keeps sleeping indefinitely until any activity
> is done with VM console.
>
> This is easily reproduce-able using kvmtool.
> ./lkvm-static run -c1 --console virtio -p "earlycon root=/dev/vda" \
>          -k ./Image -d rootfs.ext4
>
> Also, just add a print in kvm_riscv_vcpu_vstimer_expired() to
> check the interrupt delivery and run `top` or similar auto-upating
> cmd from guest. Within sometime one can notice that print from
> timer expiry routine stops and the `top` cmd output will stop
> updating.
>
> This change fixes this by making sure we schedule the timer even
> with delta_ns being zero to bring the VCPU out of sleep immediately.
>

Please add the Fixes tag here.

> Signed-off-by: Rajnesh Kanwal <rkanwal at rivosinc.com>
> ---
>  arch/riscv/kvm/vcpu_timer.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/kvm/vcpu_timer.c b/arch/riscv/kvm/vcpu_timer.c
> index ad34519c8a13..3ac2ff6a65da 100644
> --- a/arch/riscv/kvm/vcpu_timer.c
> +++ b/arch/riscv/kvm/vcpu_timer.c
> @@ -147,10 +147,8 @@ static void kvm_riscv_vcpu_timer_blocking(struct kvm_vcpu *vcpu)
>                 return;
>
>         delta_ns = kvm_riscv_delta_cycles2ns(t->next_cycles, gt, t);
> -       if (delta_ns) {
> -               hrtimer_start(&t->hrt, ktime_set(0, delta_ns), HRTIMER_MODE_REL);
> -               t->next_set = true;
> -       }
> +       hrtimer_start(&t->hrt, ktime_set(0, delta_ns), HRTIMER_MODE_REL);
> +       t->next_set = true;
>  }
>
>  static void kvm_riscv_vcpu_timer_unblocking(struct kvm_vcpu *vcpu)
> --
> 2.25.1
>