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[PATCH] xen/x86: don't lose event interrupts

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  [PATCH] xen/x86: don't lose event interrupts Stefano Stabellini <sstabellini@kernel.org> - 2016-04-16 03:30 +0200
    Re: [PATCH] xen/x86: don't lose event interrupts Juergen Gross <jgross@suse.com> - 2016-04-21 11:00 +0200
      Re: [PATCH] xen/x86: don't lose event interrupts Stefano Stabellini <sstabellini@kernel.org> - 2016-04-25 12:30 +0200

#1380459 — [PATCH] xen/x86: don't lose event interrupts

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@kernel.org>

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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#1383963

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@kernel.org>

Acked-by: Juergen Gross <jgross@suse.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;
>  }
> 

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#1386215

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@kernel.org>
> 
> Acked-by: Juergen Gross <jgross@suse.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;
> >  }
> > 
> 

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