linux-kernel - Re: [PATCH 09/22] xen: Find an unbound irq number in reverse order (high to low).

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Message-ID: <alpine.DEB.2.00.1010071130410.2440@kaball-desktop>
Date:	Thu, 7 Oct 2010 11:38:49 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>
Subject: Re: [PATCH 09/22] xen: Find an unbound irq number in reverse order
 (high to low).

On Wed, 6 Oct 2010, Konrad Rzeszutek Wilk wrote:
> > 
> > Unfortunately this is the wrong way to fix the issue: Xen has a range of
> > allowed pirq for each domain and we don't know exactly what is the
> > maximum pirq (see my patch "xen: get the maximum number of pirqs from
> > xen" [1]).
> 
> > Considering that we might use the irq number returned by
> > find_unbound_irq through xen_allocate_pirq as pirq number in some cases,
> 
> Ah, but we wouldn't! We would end up only using the 'find_unbound_irq' for
> event channels. For IRQs that are for physical devices (either being
> real devices passed in or QEMU PCI devices) we end up requesting an IRQ that
> matches whatever the device has defined in dev->irq (or whatever the
> vectors values for MSI/MSI-X devices that is provided) via the Xen PCI frontend
> driver (in case of QEMU whatever its emulation provides).
> 
> > starting from the highest value could be unsafe.
> > In practice it should be impossible to see this issue because it can
> > only happen if the irq returned by xen_allocate_pirq is higher than the
> > max pirq in xen. However AFAIK when we call xen_allocate_pirq with the
> > intention of using the return value as pirq we always fall in the if
> > (identity_mapped_irq(gsi) || !xen_initial_domain()) that avoid calling
> > find_unbound_irq.
> 
> Right, and we end up using an the pirq/gsi number at that point. This
> patch would not touch that logic.

What about adding a comment on top of xen_allocate_pirq like the
following:

/* xen_allocate_irq might allocate irqs from the top down, as a
 * consequence don't assume that the irq number returned has a low value
 * or can be used as a pirq number unless you know otherwise.
 *
 * One notable exception is when xen_allocate_irq is called passing an
 * hardware gsi as argument, in that case the irq number returned
 * matches the gsi number passed as first argument.
 */
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