linux-kernel - Re: [PATCH] xen: reuse the same pirq allocated when driver load first time

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Date:	Mon, 13 May 2013 12:06:43 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Zhenzhong Duan <zhenzhong.duan@...cle.com>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Yuval Shaia <yuval.shaia@...cle.com>,
	Feng Jin <joe.jin@...cle.com>, Chien Yen <chien.yen@...cle.com>
Subject: Re: [PATCH] xen: reuse the same pirq allocated when driver load
 first time

On Fri, 10 May 2013, Konrad Rzeszutek Wilk wrote:
> On Wed, May 08, 2013 at 04:18:24PM +0800, Zhenzhong Duan wrote:
> > When driver load and unload in a loop, pirq will exhaust finally.
> > Try to use the same pirq which was already mapped and binded at first time
> 
> So what happens if I unload and reload two drivers in random order?
> 
> > when driver loaded.
> > 
> > Read pirq from msix entry and test if data is XEN_PIRQ_MSI_DATA
> > xen_irq_from_pirq(pirq) < 0 checking is wrong as irq will be freed
> > when driver unload, it's always true in second load.
> 
> If my understanding is right the issue at hand is that the caching
> information about the pirq disappears once the driver has been
> unloaded b/c the event's irq-info is removed (as the driver is
> unloaded and free_irq is called).
> 
> Stefano,
> Is there a specific write to the MSI structure that would cause the
> hypervisor to drop the PIRQ? Or a nice hypercall to "free" an
> PIRQ in usage?

We already have a "free PIRQ" hypercall, it's called
PHYSDEVOP_unmap_pirq and should be called by QEMU.
Linux should disable the MSI bit in the PCI config options of the
device:

pci_disable_msi -> pci_msi_shutdown -> msi_set_enable(0)

That should cause QEMU to issue a xc_physdev_unmap_pirq that actually
unmaps the pirq. If it doesn't, it's a bug :)


> I presume that on the first load the msg.data value is
> ALWAYS != XEN_PIRQ_MSI_DATA right? Is that somethign we can ALWAYS
> depend? Or is there a possibility that it could be that when the
> PCI device is unplugged from one guest and plugged in another?
> 
> Could we use PHYSDEVOP_irq_status_query to figure out if the PIRQ
> has been already allocated? Ah, does not look like it.
> 
> I am a bit worried about depending on just msg.data to tells us
> whether the device had been initialized before.

Data comes from QEMU, I think it is zeroed at init time.
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