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

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Date:	Mon, 25 Apr 2016 11:28:20 +0100 (BST)
From:	Stefano Stabellini <sstabellini@...nel.org>
To:	Juergen Gross <jgross@...e.com>
cc:	Stefano Stabellini <sstabellini@...nel.org>,
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
	boris.ostrovsky@...cle.com, david.vrabel@...rix.com
Subject: Re: [PATCH] xen/x86: don't lose event interrupts

On Thu, 21 Apr 2016, Juergen Gross wrote:
> 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>

I committed it to xentip/for-linus-4.7


> > 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;
> >  }
> > 
>