linux-kernel - Re: [PATCH 9/10] Vmi timer update.patch

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Message-ID: <461BC373.10306@vmware.com>
Date:	Tue, 10 Apr 2007 10:03:47 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Chris Wright <chrisw@...s-sol.org>
CC:	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@....de>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dan Hecht <dhecht@...are.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 9/10] Vmi timer update.patch

Chris Wright wrote:


Thanks for the review!  Comments inline.

>> +/* paravirt_ops.get_wallclock = vmi_get_wallclock */
>>     
>
> Style nit, these pv_ops.foo = vmi_foo style comments aren't really useful.
>
>   

Yeah, and easy to get out of sync.  I'll drop them.

>> +	.rating         = 1000,
>>     
>
> Heh, no messing around ;-)
>   

Yes, VMI has 1000 hps.


>> +	printk(KERN_WARNING "vmi: registering clock event %s. mult=%lu shift=%u\n", 
>> +	       evt->name, evt->mult, evt->shift);
>>     
>
> Why is this a warning? ;-)
>   

Debug info, I can remove it.

>> +void __init vmi_time_init(void)
>> +{
>> +	/* Disable PIT: BIOSes start PIT CH0 with 18.2hz peridic. */
>> +	outb_p(0x3a, PIT_MODE); /* binary, mode 5, LSB/MSB, ch 0 */
>>     
>
> That shouldn't be necessary using clockevents.
>   

Actually, I'm not so sure.  If clockevents simply masks the PIT when 
disabling it, we still have overhead of keeping the latch in sync, which 
requires a timer at the PIT frequency.  I can instrument to see how 
exactly the PIT gets disabled.


>> +	vmi_time_init_clockevent();
>> +	setup_irq(0, &vmi_clock_action);
>> +}
>> +
>> +#ifdef CONFIG_X86_LOCAL_APIC
>> +void __devinit vmi_time_bsp_init(void)
>> +{
>> +	/*
>> +	 * On APIC systems, we want local timers to fire on each cpu.  We do
>> +	 * this by programming LVTT to deliver timer events to the IRQ handler
>> +	 * for IRQ-0, since we can't re-use the APIC local timer handler
>> +	 * without interfering with that code.
>> +	 */
>> +	clockevents_notify(CLOCK_EVT_NOTIFY_SUSPEND, NULL);
>>     
>
> Why do you do this suspend...
>   

We need to cancel all pending PIT timer events and restart then local 
timer, which requires atomically taking over IRQ-0.  We use the IDT gate 
for IRQ-0 because it is already an exclusive interrupt, but we can't 
re-use the LVTT IDT gate for local timer since that requires a custom 
custom SMP interrupt in entry.S.  So we must be absolutely sure when we 
get an interrupt on IRQ-0 that it came from the VMI local (rather than 
PIT) delivery path.

>   
>> +	local_irq_disable();
>> +#ifdef CONFIG_X86_SMP
>> +	/*
>> +	 * XXX handle_percpu_irq only defined for SMP; we need to switch over
>> +	 * to using it, since this is a local interrupt, which each CPU must
>> +	 * handle individually without locking out or dropping simultaneous
>> +	 * local timers on other CPUs.  We also don't want to trigger the
>> +	 * quirk workaround code for interrupts which gets invoked from
>> +	 * handle_percpu_irq via eoi, so we use our own IRQ chip.
>> +	 */
>> +	set_irq_chip_and_handler_name(0, &vmi_chip, handle_percpu_irq, "lvtt");
>> +#else
>> +	set_irq_chip_and_handler_name(0, &vmi_chip, handle_edge_irq, "lvtt");
>> +#endif
>> +	vmi_wiring = VMI_ALARM_WIRED_LVTT;
>> +	apic_write(APIC_LVTT, vmi_get_timer_vector());
>>     
>
> isn't this just your ->startup?
>   

Which structure has a ->startup function we can use?  Sorry if this 
seems ignorant, I'm not quite sure what you mean.

>   
>> +	local_irq_enable();
>> +	clockevents_notify(CLOCK_EVT_NOTIFY_RESUME, NULL);
>>     
>
> ...and resume?  Instead of letting clockevents core handle all of that,
> and just registering right here?
>   

It wasn't clear that clockevents would issue a resume notify for us; if 
so we could handle this setup in the callback, but it has to be done on 
the correct CPU.  I can try it and see if that works.

Thanks,

Zach
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