linux-kernel - Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable

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Message-ID: <46009E42.30400@vmware.com>
Date:	Tue, 20 Mar 2007 18:53:54 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@....de>,
	David Miller <davem@...emloft.net>, mingo@...e.hu,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org, xen-devel@...ts.xensource.com,
	chrisw@...s-sol.org, anthony@...emonkey.ws, netdev@...r.kernel.org
Subject: Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops
 callsites to make them patchable

Linus Torvalds wrote:
> On Tue, 20 Mar 2007, Zachary Amsden wrote:
>   
>> void local_irq_restore(int enabled)
>> {
>>    pda.intr_mask = enabled;
>>    /*
>>     * note there is a window here where softirqs are not processed by
>>     * the interrupt handler, but that is not a problem, since it will
>>     * get done here in the outer enable of any nested pair.
>>     */
>>    if (enabled)
>>        local_bh_enable();
>> }
>>     
>
> Actually, this one is more complicated. You also need to actually enable 
> hardware interrupts again if they got disabled by an interrupt actually 
> occurring while the "soft-interrupt" was disabled.
>   

Actually, I was thinking the irq handlers would just not mess around 
with eflags on the stack, just call the chip to ack the interrupt and 
re-enable hardware interrupts when they left, since that is free anyway 
with the iret.  Maybe leaving irqs disabled is better.

> Anyway, it really *should* be pretty damn simple. No need to disable 
> preemption, there should be no events that can *cause* it, since all 
> interrupts get headed off at the pass.. (the return-from-interrupt thng 
> should already notice that it's returning to an interrupts-disabled 
> section and not try to do any preemption).
>   
> What did I miss?
>   

I wasn't disabling preemption to actually disable preemption.  I was 
just using bh_disable as a global hammer to stop softirqs (thus the irq 
replay tasklet) from running during the normal irq_exit path.  Then, we 
can just use the existing software IRQ replay code, and I think barely 
any new code (queue_irq(), etc) has to be written.

Zach
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