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

[Stefano Stabellini]

On Wed, 3 May 2017, Greg KH wrote:
> On Wed, May 03, 2017 at 03:59:15PM -0700, Stefano Stabellini wrote:
> > On Wed, 3 May 2017, Boris Ostrovsky wrote:
> > > On 05/03/2017 02:19 PM, David Woodhouse wrote:
> > > > On Wed, 2017-02-22 at 10:14 -0500, Boris Ostrovsky wrote:
> > > >> On 02/22/2017 09:28 AM, Bjorn Helgaas wrote:
> > > >>> On Tue, Feb 21, 2017 at 10:58:39AM -0500, Boris Ostrovsky wrote:
> > > >>>> On 02/21/2017 10:45 AM, Juergen Gross wrote:
> > > >>>>> On 21/02/17 16:31, Dan Streetman wrote:
> > > >>>>>> On Fri, Jan 13, 2017 at 5:30 PM, Konrad Rzeszutek Wilk
> > > >>>>>> <konrad.wilk@xxxxxxxxxx> wrote:
> > > >>>>>>> On Fri, Jan 13, 2017 at 03:07:51PM -0500, Dan Streetman wrote:
> > > >>>>>>>> Revert the main part of commit:
> > > >>>>>>>> af42b8d12f8a ("xen: fix MSI setup and teardown for PV on HVM guests")
> > > >>>>>>>>
> > > >>>>>>>> That commit introduced reading the pci device's msi message data to see
> > > >>>>>>>> if a pirq was previously configured for the device's msi/msix, and re-use
> > > >>>>>>>> that pirq.  At the time, that was the correct behavior.  However, a
> > > >>>>>>>> later change to Qemu caused it to call into the Xen hypervisor to unmap
> > > >>>>>>>> all pirqs for a pci device, when the pci device disables its MSI/MSIX
> > > >>>>>>>> vectors; specifically the Qemu commit:
> > > >>>>>>>> c976437c7dba9c7444fb41df45468968aaa326ad
> > > >>>>>>>> ("qemu-xen: free all the pirqs for msi/msix when driver unload")
> > > >>>>>>>>
> > > >>>>>>>> Once Qemu added this pirq unmapping, it was no longer correct for the
> > > >>>>>>>> kernel to re-use the pirq number cached in the pci device msi message
> > > >>>>>>>> data.  All Qemu releases since 2.1.0 contain the patch that unmaps the
> > > >>>>>>>> pirqs when the pci device disables its MSI/MSIX vectors.
> > > >>>>>>>>
> > > >>>>>>>> This bug is causing failures to initialize multiple NVMe controllers
> > > >>>>>>>> under Xen, because the NVMe driver sets up a single MSIX vector for
> > > >>>>>>>> each controller (concurrently), and then after using that to talk to
> > > >>>>>>>> the controller for some configuration data, it disables the single MSIX
> > > >>>>>>>> vector and re-configures all the MSIX vectors it needs.  So the MSIX
> > > >>>>>>>> setup code tries to re-use the cached pirq from the first vector
> > > >>>>>>>> for each controller, but the hypervisor has already given away that
> > > >>>>>>>> pirq to another controller, and its initialization fails.
> > > >>>>>>>>
> > > >>>>>>>> This is discussed in more detail at:
> > > >>>>>>>> https://lists.xen.org/archives/html/xen-devel/2017-01/msg00447.html
> > > >>>>>>>>
> > > >>>>>>>> Fixes: af42b8d12f8a ("xen: fix MSI setup and teardown for PV on HVM guests")
> > > >>>>>>>> Signed-off-by: Dan Streetman <dan.streetman@xxxxxxxxxxxxx>
> > > >>>>>>> Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > > >>>>>> This doesn't seem to be applied yet, is it still waiting on another
> > > >>>>>> ack?  Or maybe I'm looking at the wrong git tree...
> > > >>>>> Am I wrong or shouldn't this go through the PCI tree? Konrad?
> > > >>>> Konrad is away this week but since pull request for Xen tree just went
> > > >>>> out we should probably wait until rc1 anyway (unless something big comes
> > > >>>> up before that).
> > > >>> I assume this should go via the Xen or x86 tree, since that's how most
> > > >>> arch/x86/pci/xen.c patches have been handled, including af42b8d12f8a.
> > > >>> If you think otherwise, let me know.
> > > >> OK, I applied it to Xen tree's for-linus-4.11.
> > > > Hm, we want this (c74fd80f2f4) in stable too, don't we?
> > > 
> > > 
> > > Maybe.
> > > 
> > > Per https://lists.xen.org/archives/html/xen-devel/2017-01/msg00987.html
> > > it may break things on older (4.4-) hypervisors. They are out of
> > > support, which is why this patch went in now but I am not sure this
> > > automatically applies to stable kernels.
> > > 
> > > Stefano?
> > 
> > This is a difficult call. We could just say that all the broken Xen
> > versions are out of support, so let's fix all the Linux kernel stable
> > trees that we can.
> > 
> > Or we could give a look at the release dates. Linux 3.18 is still
> > maintained and was tagged on Dec 7 2014.
> 
> Don't do anything "special" for 3.18 if you have to.  I'm only
> semi-maintaining it because some SoC vendors never upstreamed their
> trees and lots of devices rely on it.  None of them use Xen on their
> platforms, so no need for me to backport any change there.

Thanks Greg, that is good info. Is 4.4 the oldest Linux tree fully
maintained?

If so, I think we should just backport this fix to all Linux trees >=
4.4, given that Linux 4.4 is from Jan 2016.