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

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Date:	Thu, 7 Oct 2010 10:05:26 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	Fitzhardinge <jeremy@...p.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Jeremy@...inet11.oracle.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>
Subject: Re: [Xen-devel] Re: [PATCH 09/22] xen: Find an unbound irq number
 in reverse order (high to low).

On Thu, Oct 07, 2010 at 11:38:49AM +0100, Stefano Stabellini wrote:
> 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.
>  */

Done!
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