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Date: Tue, 20 Mar 2007 18:00:25 +0100 From: Ingo Molnar <mingo@...e.hu> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Jeremy Fitzhardinge <jeremy@...p.org>, Zachary Amsden <zach@...are.com>, Rusty Russell <rusty@...tcorp.com.au>, "Eric W. Biederman" <ebiederm@...ssion.com>, Andi Kleen <ak@....de>, David Miller <davem@...emloft.net>, 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 <torvalds@...ux-foundation.org> wrote: > I have this dim memory that ARM has done it that way for a long time > because it's so expensive to do a "real" cli/sti. > > And I think -rt does it for other reasons. It's just more flexible. -rt doesnt wrap cli/sti anymore: spin_lock_irq*() doesnt disable irq flags on -rt, thus the amount of real irqs-off sections is very small and reviewable. But nevertheless an incarnation of the code survived and is upstream already, in the form of TRACE_IRQFLAGS lockdep code ;) This implements a soft hardirq flag _today_: all that would be needed is for Xen to define raw_local_irq_disable() as a NOP, and to use the current->hardirqs_enabled as 'soft IRQ-off flag'. Note that ->hardirqs_enabled is self-maintained, i.e. it's not just a stupid shadow of the hardirq flag, it's an independently maintained flag that does not rely on the existence of the hard flag. [ this code even has its own debugging code, so out-of-sync-flags, double-off and double-on is detected and complained about. So all the hard stuff has already been done as part of lockdep, and it's even long-term maintainable because under a native lockdep kernel we check the soft flag against the hard flag. ] so i think a soft cli/sti flag support should be merged/unified with the trace_hardirqs_on()/trace_hardirqs_off() code. Ingo - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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