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Date: Mon, 13 May 2013 14:20:55 -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 06:24:46PM +0100, Stefano Stabellini wrote: > On Mon, 13 May 2013, Konrad Rzeszutek Wilk wrote: > > 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. > > QEMU is the owner of the pirq, in fact it is the one that creates and > destroys the mapping. I think that the right place to fix this problem > is in QEMU, the ABI would be much cleaner as a result. As a side effect > we don't need to make any changes in Linux. You do. You need to remove the PHYSDEVOP_get_free_pirq call in that case. > > > > > > > > > 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. > > We could do that and it might be easier than changing QEMU, but I think > it would be more clear and consistent if QEMU was the one to do the > unmapping. And allocating the PIRQ? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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