Re: [PATCH] xen: always handle VIRQ_TIMER first.

[Jeremy Fitzhardinge <jeremy@goop.org>]

 On 10/15/2010 03:52 AM, Ian Campbell wrote:
> This ensures that system is updated before calling any hard irq
> handlers after a long period of ticklessness. If we do not do this
> then hardirq will see a jiffies from before the period of ticklessness
> and make intcorrect decisions regarding timer expiry etc.
>
> This resolves issues e.g. with USB keyboard timer repeats.
>
> Based on a patch by Keir Fraser.

I talked about this with James, and it makes no sense to me at all.

    J

> Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
> Cc: keir@xen.org
> ---
>  drivers/xen/events.c |   22 +++++++++++++++++++++-
>  1 files changed, 21 insertions(+), 1 deletions(-)
>
> diff --git a/drivers/xen/events.c b/drivers/xen/events.c
> index c9d1d4a..1496ba5 100644
> --- a/drivers/xen/events.c
> +++ b/drivers/xen/events.c
> @@ -1052,6 +1052,7 @@ static void __xen_evtchn_do_upcall(struct pt_regs *regs)
>  
>  	do {
>  		unsigned long pending_words;
> +		int irq;
>  
>  		vcpu_info->evtchn_upcall_pending = 0;
>  
> @@ -1062,6 +1063,24 @@ static void __xen_evtchn_do_upcall(struct pt_regs *regs)
>  		/* Clear master flag /before/ clearing selector flag. */
>  		wmb();
>  #endif
> +
> +		/*
> +		 * Handle timer interrupts before all others, so that all
> +		 * hardirq handlers see an up-to-date system time even if we
> +		 * have just woken from a long idle period.
> +		 */
> +		irq = percpu_read(virq_to_irq[VIRQ_TIMER]);
> +		if (irq != -1) {
> +			int word_idx;
> +			int bit_idx;
> +			int port = evtchn_from_irq(irq);
> +			word_idx = port / BITS_PER_LONG;
> +			bit_idx = port % BITS_PER_LONG;
> +			if (VALID_EVTCHN(port) &&
> +			    (active_evtchns(cpu, s, word_idx) & (1UL<<bit_idx)))
> +				(void)handle_irq(irq, regs);
> +		}
> +
>  		pending_words = xchg(&vcpu_info->evtchn_pending_sel, 0);
>  		while (pending_words != 0) {
>  			unsigned long pending_bits;
> @@ -1071,9 +1090,10 @@ static void __xen_evtchn_do_upcall(struct pt_regs *regs)
>  			while ((pending_bits = active_evtchns(cpu, s, word_idx)) != 0) {
>  				int bit_idx = __ffs(pending_bits);
>  				int port = (word_idx * BITS_PER_LONG) + bit_idx;
> -				int irq = evtchn_to_irq[port];
>  				struct irq_desc *desc;
>  
> +				irq = evtchn_to_irq[port];
> +
>  				mask_evtchn(port);
>  				clear_evtchn(port);
>