Date:   Fri, 13 Jan 2017 10:44:03 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     jgross@...e.com, boris.ostrovsky@...cle.com
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Dan Streetman <dan.streetman@...onical.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        xen-devel@...ts.xenproject.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-pci@...r.kernel.org, ddstreet@...e.org
Subject: Re: [Xen-devel] [PATCH] xen: do not re-use pirq number cached in
 pci device msi msg data

On Fri, 13 Jan 2017, Dan Streetman wrote:
> On Wed, Jan 11, 2017 at 6:25 PM, Dan Streetman <ddstreet@...e.org> wrote:
> > On Wed, Jan 11, 2017 at 1:46 PM, Stefano Stabellini
> > <sstabellini@...nel.org> wrote:
> >> On Wed, 11 Jan 2017, Dan Streetman wrote:
> >>> On Tue, Jan 10, 2017 at 8:25 PM, Stefano Stabellini
> >>> <sstabellini@...nel.org> wrote:
> >>> > On Tue, 10 Jan 2017, Dan Streetman wrote:
> >>> >> On Tue, Jan 10, 2017 at 2:03 PM, Stefano Stabellini
> >>> >> <sstabellini@...nel.org> wrote:
> >>> >> > On Tue, 10 Jan 2017, Dan Streetman wrote:
> >>> >> >> On Tue, Jan 10, 2017 at 10:57 AM, Dan Streetman <ddstreet@...e.org> wrote:
> >>> >> >> > On Mon, Jan 9, 2017 at 2:30 PM, Stefano Stabellini
> >>> >> >> > <sstabellini@...nel.org> 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@...cle.com> 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@...citrix.com>
> >>> >> >> >> 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@...cle.com>
> >> 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. 

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