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

[Dan Streetman]

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...