linux-kernel - Re: [PATCH] xen/x86: don't lose event interrupts

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Date:	Thu, 21 Apr 2016 10:59:05 +0200
From:	Juergen Gross <jgross@...e.com>
To:	Stefano Stabellini <sstabellini@...nel.org>,
	xen-devel@...ts.xenproject.org
Cc:	linux-kernel@...r.kernel.org, boris.ostrovsky@...cle.com,
	david.vrabel@...rix.com
Subject: Re: [PATCH] xen/x86: don't lose event interrupts

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@...nel.org>

Acked-by: Juergen Gross <jgross@...e.com>

> 
> 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;
>  }
>