Re: [Xen-devel] [PATCH] xen/x86: don't lose event interrupts

[Juergen Gross <jgross@xxxxxxxx>]

On 16/04/16 03:23, Stefano Stabellini wrote:
> On slow platforms with unreliable TSC, such as QEMU emulated machines,
> it is possible for the kernel to request the next event in the past. In
> that case, in the current implementation of xen_vcpuop_clockevent, we
> simply return -ETIME. To be precise the Xen returns -ETIME and we pass
> it on. However the result of this is a missed event, which simply causes
> the kernel to hang.
> 
> Instead it is better to always ask the hypervisor for a timer event,
> even if the timeout is in the past. That way there are no lost
> interrupts and the kernel survives. To do that, remove the
> VCPU_SSHOTTMR_future flag.
> 
> Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>

Acked-by: Juergen Gross <jgross@xxxxxxxx>

> 
> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
> index a0a4e55..6deba5b 100644
> --- a/arch/x86/xen/time.c
> +++ b/arch/x86/xen/time.c
> @@ -290,11 +290,11 @@ static int xen_vcpuop_set_next_event(unsigned long 
> delta,
>       WARN_ON(!clockevent_state_oneshot(evt));
>  
>       single.timeout_abs_ns = get_abs_timeout(delta);
> -     single.flags = VCPU_SSHOTTMR_future;
> +     /* Get an event anyway, even if the timeout is already expired */
> +     single.flags = 0;
>  
>       ret = HYPERVISOR_vcpu_op(VCPUOP_set_singleshot_timer, cpu, &single);
> -
> -     BUG_ON(ret != 0 && ret != -ETIME);
> +     BUG_ON(ret != 0);
>  
>       return ret;
>  }
> 


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