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

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Message-ID: <20130513161714.GC10401@phenom.dumpdata.com>
Date:	Mon, 13 May 2013 12:17:14 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	Zhenzhong Duan <zhenzhong.duan@...cle.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 Mon, May 13, 2013 at 03:50:52PM +0100, Stefano Stabellini wrote:
> On Mon, 13 May 2013, Konrad Rzeszutek Wilk wrote:
> > On Mon, May 13, 2013 at 12:06:43PM +0100, Stefano Stabellini wrote:
> > > 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.
> > 
> > Considering that we call function that allocates (PHYSDEVOP_get_free_pirq)
> > it in the Linux kernel (and not in QEMU), perhaps that should be done in the
> > Linux kernel as part of xen_destroy_irq()? Or would that confuse QEMU?
> 
> I think it would confuse QEMU. It is probably better to let the unmap
> being handled by it.
> 
> 
> > It looks like QEMU only does that hypercall (via xc_physdev_unmap_pirq)
> > unregister_real_device which is only called during pci unplug?
> 
> You are right! I would think that this behaviour is erroneous unless it
> was done on purpose to avoid allocating MSIs twice.
> If that is the case we would need to do something similar in Linux too.
> 
> I think that the issue is the mismatch between QEMU's and Linux's
> behaviours: either both should be allocating MSIs once, or they should
> both be allocating and deallocating MSIs every time the driver is loaded
> and unloaded.

Right. But we also have the scenario that QEMU and Linux are going to 
be out of sync. So we need fixes in both places - I think.

> 
> 
> > > Linux should disable the MSI bit in the PCI config options of the
> > > device:
> > > 
> > > pci_disable_msi -> pci_msi_shutdown -> msi_set_enable(0)
> > 
> > Zhenzhong, does it do that? Looking at the driver it certainly seems that way.
> > > 
> > > That should cause QEMU to issue a xc_physdev_unmap_pirq that actually
> > > unmaps the pirq. If it doesn't, it's a bug :)
> > 
> > <sigh> It doesn't do that. So two bugs:
> >  - QEMU doing that
> >  - Linux kernel doing the hypercall as well.
> 
> At first sight I also thought that the Linux kernel was issuing
> PHYSDEVOP_unmap_pirq too but actually Linux is only doing it if
> xen_initial_domain().

That seems like an easy fix. Just do 'if (xen_initial_domain()
|| xen_hvm_domain())'. I think the only one we cannot do it for
is 'xen_pv_domain()' (so PCI in PV guests) as the "owner" of the PIRQ is
actually dom0.

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