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Re: [Xen-devel] [PATCH] xen: do not re-use pirq number cached in pci device msi msg data

On 01/13/2017 01:44 PM, Stefano Stabellini wrote: > On Fri, 13 Jan 2017, Dan Streetman wrote: >> On Wed, Jan 11, 2017 at 6:25 PM, Dan Streetman <ddstreet@xxxxxxxx> wrote: >>> On Wed, Jan 11, 2017 at 1:46 PM, Stefano Stabellini >>> <sstabellini@xxxxxxxxxx> wrote: >>>> On Wed, 11 Jan 2017, Dan Streetman wrote: >>>>> On Tue, Jan 10, 2017 at 8:25 PM, Stefano Stabellini >>>>> <sstabellini@xxxxxxxxxx> wrote: >>>>>> On Tue, 10 Jan 2017, Dan Streetman wrote: >>>>>>> On Tue, Jan 10, 2017 at 2:03 PM, Stefano Stabellini >>>>>>> <sstabellini@xxxxxxxxxx> wrote: >>>>>>>> On Tue, 10 Jan 2017, Dan Streetman wrote: >>>>>>>>> On Tue, Jan 10, 2017 at 10:57 AM, Dan Streetman <ddstreet@xxxxxxxx> >>>>>>>>> wrote: >>>>>>>>>> On Mon, Jan 9, 2017 at 2:30 PM, Stefano Stabellini >>>>>>>>>> <sstabellini@xxxxxxxxxx> wrote: >>>>>>>>>>> On Mon, 9 Jan 2017, Konrad Rzeszutek Wilk wrote: >>>>>>>>>>>> On Mon, Jan 09, 2017 at 10:42:41AM -0500, Dan Streetman wrote: >>>>>>>>>>>>> On Mon, Jan 9, 2017 at 9:59 AM, Boris Ostrovsky >>>>>>>>>>>>> <boris.ostrovsky@xxxxxxxxxx> wrote: >>>>>>>>>>>>>> On 01/06/2017 08:06 PM, Konrad Rzeszutek Wilk wrote: >>>>>>>>>>>>>>> On Thu, Jan 05, 2017 at 02:28:56PM -0500, Dan Streetman wrote: >>>>>>>>>>>>>>>> Do not read a pci device's msi message data to see if a pirq >>>>>>>>>>>>>>>> was >>>>>>>>>>>>>>>> previously configured for the device's msi/msix, as the old >>>>>>>>>>>>>>>> pirq was >>>>>>>>>>>>>>>> unmapped and may now be in use by another pci device. The >>>>>>>>>>>>>>>> previous >>>>>>>>>>>>>>>> pirq should never be re-used; instead a new pirq should always >>>>>>>>>>>>>>>> be >>>>>>>>>>>>>>>> allocated from the hypervisor. >>>>>>>>>>>>>>> Won't this cause a starvation problem? That is we will run out >>>>>>>>>>>>>>> of PIRQs >>>>>>>>>>>>>>> as we are not reusing them? >>>>>>>>>>>>>> Don't we free the pirq when we unmap it? >>>>>>>>>>>>> I think this is actually a bit worse than I initially thought. >>>>>>>>>>>>> After >>>>>>>>>>>>> looking a bit closer, and I think there's an asymmetry with pirq >>>>>>>>>>>>> allocation: >>>>>>>>>>>> Lets include Stefano, >>>>>>>>>>>> >>>>>>>>>>>> Thank you for digging in this! This has quite the deja-vu >>>>>>>>>>>> feeling as I believe I hit this at some point in the past and >>>>>>>>>>>> posted some possible ways of fixing this. But sadly I can't >>>>>>>>>>>> find the thread. >>>>>>>>>>> This issue seems to be caused by: >>>>>>>>>>> >>>>>>>>>>> commit af42b8d12f8adec6711cb824549a0edac6a4ae8f >>>>>>>>>>> Author: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx> >>>>>>>>>>> Date: Wed Dec 1 14:51:44 2010 +0000 >>>>>>>>>>> >>>>>>>>>>> xen: fix MSI setup and teardown for PV on HVM guests >>>>>>>>>>> >>>>>>>>>>> which was a fix to a bug: >>>>>>>>>>> >>>>>>>>>>> This fixes a bug in xen_hvm_setup_msi_irqs that manifests >>>>>>>>>>> itself when >>>>>>>>>>> trying to enable the same MSI for the second time: the old MSI >>>>>>>>>>> to pirq >>>>>>>>>>> mapping is still valid at this point but xen_hvm_setup_msi_irqs >>>>>>>>>>> would >>>>>>>>>>> try to assign a new pirq anyway. >>>>>>>>>>> A simple way to reproduce this bug is to assign an MSI capable >>>>>>>>>>> network >>>>>>>>>>> card to a PV on HVM guest, if the user brings down the >>>>>>>>>>> corresponding >>>>>>>>>>> ethernet interface and up again, Linux would fail to enable >>>>>>>>>>> MSIs on the >>>>>>>>>>> device. >>>>>>>>>>> >>>>>>>>>>> I don't remember any of the details. From the description of this >>>>>>>>>>> bug, >>>>>>>>>>> it seems that Xen changed behavior in the past few years: before it >>>>>>>>>>> used >>>>>>>>>>> to keep the pirq-MSI mapping, while today it doesn't. If I wrote >>>>>>>>>>> "the >>>>>>>>>>> old MSI to pirq mapping is still valid at this point", the pirq >>>>>>>>>>> couldn't >>>>>>>>>>> have been completely freed, then reassigned to somebody else the >>>>>>>>>>> way it >>>>>>>>>>> is described in this email. >>>>>>>>>>> >>>>>>>>>>> I think we should indentify the changeset or Xen version that >>>>>>>>>>> introduced >>>>>>>>>>> the new behavior. If it is old enough, we might be able to just >>>>>>>>>>> revert >>>>>>>>>>> af42b8d12f8adec6711cb824549a0edac6a4ae8f. Otherwise we could make >>>>>>>>>>> the >>>>>>>>>>> behavior conditional to the Xen version. >>>>>>>>>> Are PT devices the only MSI-capable devices available in a Xen guest? >>>>>>>>>> That's where I'm seeing this problem, with PT NVMe devices. >>>>>>>> They are the main ones. It is possible to have emulated virtio devices >>>>>>>> with emulated MSIs, for example virtio-net. Althought they are not in >>>>>>>> the Xen Project CI-loop, so I wouldn't be surprised if they are broken >>>>>>>> too. >>>>>>>> >>>>>>>> >>>>>>>>>> I can say that on the Xen guest with NVMe PT devices I'm testing on, >>>>>>>>>> with the patch from this thread (which essentially reverts your >>>>>>>>>> commit >>>>>>>>>> above) as well as some added debug to see the pirq numbers, cycles of >>>>>>>>>> 'modprobe nvme ; rmmod nvme' don't cause pirq starvation, as the >>>>>>>>>> hypervisor provides essentially the same pirqs for each modprobe, >>>>>>>>>> since they were freed by the rmmod. >>>>>>>> I am fine with reverting the old patch, but we need to understand what >>>>>>>> caused the change in behavior first. Maybe somebody else with a Xen PCI >>>>>>>> passthrough setup at hand can help with that. >>>>>>>> >>>>>>>> In the Xen code I can still see: >>>>>>>> >>>>>>>> case ECS_PIRQ: { >>>>>>>> struct pirq *pirq = pirq_info(d1, chn1->u.pirq.irq); >>>>>>>> >>>>>>>> if ( !pirq ) >>>>>>>> break; >>>>>>>> if ( !is_hvm_domain(d1) ) >>>>>>>> pirq_guest_unbind(d1, pirq); >>>>>>>> >>>>>>>> which means that pirq_guest_unbind should only be called on >>>>>>>> evtchn_close >>>>>>>> if the guest is not an HVM guest. >>>>>>> I tried an experiment to call get_free_pirq on both sides of a >>>>>>> evtchn_close hcall, using two SRIOV nics. When I rmmod/modprobe, I >>>>>>> see something interesting; each nic uses 3 MSIs, and it looks like >>>>>>> when they shut down, each nic's 3 pirqs are not available until after >>>>>>> the nic is done shutting down, so it seems like closing the evtchn >>>>>>> isn't what makes the pirq free. >>>>>>> >>>>>>> [3697700.390188] xen:events: creating evtchn using pirq 101 irq 290 >>>>>>> [3697700.390214] xen:events: creating evtchn using pirq 100 irq 291 >>>>>>> [3697700.390228] xen:events: creating evtchn using pirq 99 irq 292 >>>>>>> [3697700.392789] ixgbevf 0000:00:03.0: NIC Link is Up 10 Gbps >>>>>>> [3697700.406167] xen:events: creating evtchn using pirq 98 irq 293 >>>>>>> [3697700.406222] xen:events: creating evtchn using pirq 97 irq 294 >>>>>>> [3697700.406259] xen:events: creating evtchn using pirq 96 irq 295 >>>>>>> [3697700.408345] ixgbevf 0000:00:04.0: NIC Link is Up 10 Gbps >>>>>>> >>>>>>> nic 3 uses pirq 99-101, while nic 4 uses pirq 96-98. >>>>>>> >>>>>>> [3697705.470151] xen:events: shutdown_pirq: xen_domain() == 1, >>>>>>> xen_pv_domain() == 0, xen_hvm_domain() == 1, xen_initial_domain() == >>>>>>> 0, xen_pvh_domain() == 0 >>>>>>> >>>>>>> just to be sure, a bit of dbg so I know what domain this is :-) >>>>>>> >>>>>>> [3697778.781463] xen:events: shutdown_pirq: get_free_pirq returned pirq >>>>>>> 93 >>>>>>> [3697778.781465] xen:events: shutdown_pirq: closing evtchn for pirq 96 >>>>>>> irq 295 >>>>>>> [3697778.781475] xen:events: shutdown_pirq: get_free_pirq returned pirq >>>>>>> 92 >>>>>>> [3697778.781489] xen:events: shutdown_pirq: get_free_pirq returned pirq >>>>>>> 91 >>>>>>> [3697778.781490] xen:events: shutdown_pirq: closing evtchn for pirq 97 >>>>>>> irq 294 >>>>>>> [3697778.781498] xen:events: shutdown_pirq: get_free_pirq returned pirq >>>>>>> 90 >>>>>>> [3697778.781508] xen:events: shutdown_pirq: get_free_pirq returned pirq >>>>>>> 89 >>>>>>> [3697778.781509] xen:events: shutdown_pirq: closing evtchn for pirq 98 >>>>>>> irq 293 >>>>>>> [3697778.781517] xen:events: shutdown_pirq: get_free_pirq returned pirq >>>>>>> 88 >>>>>>> >>>>>>> nic 4 is shutdown first, and closes its evtchns for pirqs 96-98; but >>>>>>> none of those become available for get_free_pirq. >>>>>>> >>>>>>> [3697779.005501] xen:events: shutdown_pirq: get_free_pirq returned pirq >>>>>>> 98 >>>>>>> >>>>>>> aha, now nic 4 has fully finished shutting down, and nic 3 has started >>>>>>> shutdown; we see those pirqs from nic 4 are now available. So it must >>>>>>> not be evtchn closing that frees the pirqs. >>>>>>> >>>>>>> [3697779.005503] xen:events: shutdown_pirq: closing evtchn for pirq 99 >>>>>>> irq 292 >>>>>>> [3697779.005512] xen:events: shutdown_pirq: get_free_pirq returned pirq >>>>>>> 97 >>>>>>> [3697779.005524] xen:events: shutdown_pirq: get_free_pirq returned pirq >>>>>>> 96 >>>>>>> [3697779.005526] xen:events: shutdown_pirq: closing evtchn for pirq 100 >>>>>>> irq 291 >>>>>>> [3697779.005540] xen:events: shutdown_pirq: get_free_pirq returned pirq >>>>>>> 87 >>>>>>> [3697779.005611] xen:events: shutdown_pirq: get_free_pirq returned pirq >>>>>>> 86 >>>>>>> [3697779.005624] xen:events: shutdown_pirq: closing evtchn for pirq 101 >>>>>>> irq 290 >>>>>>> [3697779.005659] xen:events: shutdown_pirq: get_free_pirq returned pirq >>>>>>> 85 >>>>>>> >>>>>>> >>>>>>> so, since pci_disable_msix eventually calls xen_teardown_msi_irq() >>>>>>> which calls xen_destroy_irq(), i moved the dbg to xen_destroy_irq() >>>>>>> (and recompiled/rebooted) and found the pirqs have already been freed >>>>>>> before that is called: >>>>>>> >>>>>>> [3700084.714686] xen:events: shutdown_pirq: closing evtchn for pirq 98 >>>>>>> irq 295 >>>>>>> [3700084.714702] xen:events: shutdown_pirq: closing evtchn for pirq 99 >>>>>>> irq 294 >>>>>>> [3700084.714708] xen:events: shutdown_pirq: closing evtchn for pirq 100 >>>>>>> irq 293 >>>>>>> [3700084.775598] xen:events: xen_destroy_irq: get_free_pirq returned >>>>>>> pirq 100 >>>>>>> [3700084.775599] xen:events: xen_destroy_irq: pirq 100 irq 293 >>>>>>> [3700084.775624] xen:events: xen_destroy_irq: get_free_pirq returned >>>>>>> pirq 99 >>>>>>> [3700084.775631] xen:events: xen_destroy_irq: get_free_pirq returned >>>>>>> pirq 98 >>>>>>> [3700084.775632] xen:events: xen_destroy_irq: pirq 99 irq 294 >>>>>>> [3700084.775646] xen:events: xen_destroy_irq: get_free_pirq returned >>>>>>> pirq 97 >>>>>>> [3700084.775653] xen:events: xen_destroy_irq: get_free_pirq returned >>>>>>> pirq 96 >>>>>>> [3700084.775654] xen:events: xen_destroy_irq: pirq 98 irq 295 >>>>>>> [3700084.775666] xen:events: xen_destroy_irq: get_free_pirq returned >>>>>>> pirq 95 >>>>>>> >>>>>>> >>>>>>> I'm still following thru the kernel code, but it's not immediately >>>>>>> obvious what exactly is telling the hypervisor to free the pirqs; any >>>>>>> idea? >>>>>>> >>>>>>> >From the hypervisor code, it seems that the pirq is "available" via >>>>>>> is_free_pirq(): >>>>>>> return !pirq || (!pirq->arch.irq && (!is_hvm_domain(d) || >>>>>>> pirq->arch.hvm.emuirq == IRQ_UNBOUND)); >>>>>>> >>>>>>> when the evtchn is closed, it does: >>>>>>> if ( is_hvm_domain(d1) && domain_pirq_to_irq(d1, pirq->pirq) > >>>>>>> 0 ) >>>>>>> unmap_domain_pirq_emuirq(d1, pirq->pirq); >>>>>>> >>>>>>> and that call to unmap_domain_pirq_emuirq does: >>>>>>> info->arch.hvm.emuirq = IRQ_UNBOUND; >>>>>>> >>>>>>> so, the only thing left is to clear pirq->arch.irq,but the only place >>>>>>> I can find that does that is clear_domain_irq_pirq(), which is only >>>>>>> called from pirq_guest_unbind() and unmap_domain_pirq(), but I'm not >>>>>>> seeing where either of those would be called when all the kernel is >>>>>>> doing is disabling a pci device. >>>>>> Thanks for the info. I think I know what causes the pirq to be unmapped: >>>>>> when Linux disables msi or msix on the device, using the regular pci >>>>>> config space based method, QEMU (which emulates the PCI config space) >>>>>> tells Xen to unmap the pirq. >>>>> aha, via a XEN_DOMCTL_unbind_pt_irq, maybe? Well that makes more sense >>>>> then. >>>>> >>>>>> If that's the case, and if it isn't possible for xen_hvm_setup_msi_irqs >>>>>> to be called a second time without msis being disabled first, then I >>>>>> think we can revert the patch. >>>>> It doesn't seem possible to call it twice from a correctly-behaved >>>>> driver, but in case of a driver bug that does try to enable msi/msix >>>>> multiple times without disabling, __pci_enable_msix() only does >>>>> WARN_ON(!!dev->msix_enabled), and __pci_enable_msi_range() only does >>>>> WARN_ON(!!dev->msi_enabled); they both will continue. Maybe that >>>>> should be changed to warn and also return error, to prevent >>>>> re-configuring msi/msix if already configured? Or, maybe the warning >>>>> is enough - the worst thing that reverting the patch does is use extra >>>>> pirqs, right? >>>> I think the warning is enough. Can you confirm that with >>>> af42b8d12f8adec6711cb824549a0edac6a4ae8f reverted, also >>>> >>>> ifconfig eth0 down; ifconfig eth0 up >>>> >>>> still work as expected, no warnings? >>> yes, with the patch that started this thread - which essentially >>> reverts af42b8d12f8adec6711cb824549a0edac6a4ae8f - there are no >>> warnings and ifconfig down ; ifconfig up work as expected. >>> >>>> >>>> It looks like the patch that changed hypervisor (QEMU actually) behavior >>>> is: >>>> >>>> commit c976437c7dba9c7444fb41df45468968aaa326ad >>>> Author: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx> >>>> Date: Wed May 7 13:41:48 2014 +0000 >>>> >>>> qemu-xen: free all the pirqs for msi/msix when driver unload >>>> >>>> From this commit onward, QEMU started unmapping pirqs when MSIs are >>>> disabled. c976437c7dba9c7444fb41df45468968aaa326ad is present in 4.8, >>>> 4.7, 4.6, 4.5. The newest release without the commit is Xen 4.4. >>>> >>>> If we revert af42b8d12f8adec6711cb824549a0edac6a4ae8f, we fix the bug on >>>> all Xen versions from 4.5 onward, but we break the behavior on Xen 4.4 >>>> and older. Given that Xen 4.4 is out of support, I think we should go >>>> ahead with it. Opinions? >> Looks like there's no complaints; is my patch from the start of this >> thread ok to use, or can you craft a patch to use? My patch's >> description could use updating to add some of the info/background from >> this discussion... > Hi Dan, I would like an explicit Ack from the other maintainers, Boris > and Juergen. Let me place them in To: to make it more obvious. Where is the patch? I don't think 'git revert' will work. And Konrad will need to ack it too as he is Xen-PCI maintainer. -boris _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx https://lists.xen.org/xen-devel

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