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

[Dan Streetman <ddstreet@xxxxxxxx>]

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?

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