[Bug 1563375] Re: A a single PCI read or write appears twice on the PCIe bus. This happens when using the SR-IOV feature with Cavium's liquidIO card

Ryan Harper 1563375 at bugs.launchpad.net
Wed Jul 27 14:11:48 UTC 2016


I've opened a public bug without vendor details here:

https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1606940

We'll use that bug to track the SRU into trusty.

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Title:
  A a single PCI read or write appears twice on the PCIe bus. This
  happens when using the SR-IOV feature with  Cavium's liquidIO card

Status in linux package in Ubuntu:
  Invalid
Status in qemu package in Ubuntu:
  Triaged
Status in linux source package in Trusty:
  Invalid
Status in qemu source package in Trusty:
  Triaged

Bug description:
  [Impact]

   * Users of SRIOV devices in qemu on Trusty may encounter unstable
     behavior on pass-through PCI devices due to a bug in qemu's MMIO
     mapping to overlapping ram slots.  When memory is accessed in
     subpage granularity where slots have overlapping regions multiple
     invocations of the handler ocurrs which resulted in multiple pci
     writes.
     
     This affects the qemu releases prior to qemu 2.5, it has been fixed in
     newer releases. 
     
   * Backporting fixes from upstream release is required to allow
     certain PCI devices under SRIOV to function properly.

   * All patches applied are already accepted upstream.  Xenial, Yakkety
     are OK, Wily -> Trusty are affected.
     

  [Test Case]

   * On a Trusty 14.04 system with affected SRIOV device.
      - boot system with sriov enabled
      - launch vm with sriov device passed through
        using guest XML attached (bug-1563375-trusty-guest.xml)
      - unpack pcimem tarball inside vm (pcimem.tar attached)
      - Read (note the pci path should point to the SRIOV device)
       ./pcimem /sys/bus/pci/devices/0000\:04\:00.0/resource0 0x10080 d
      - Write
       ./pcimem /sys/bus/pci/devices/0000\:04\:00.0/resource0 0x10080 d 2048
      - Read again
       ./pcimem /sys/bus/pci/devices/0000\:04\:00.0/resource0 0x10080 d

      The value of SLIST_BAOFF_DBELL should be the same for the first read
      and the second read, after the write.

      If the bug is hit, the second read will report a value of double
      instead of the same.

  [Regression Potential]

   * SR-IOV device drivers may have unknowingly relied on KVM multi-write
     behavior prior to this patch; that's highly unlikely since it would
     fail on physical hardware (which does not produce this effect).  But
     there is a chance that devices only passed into the guest via SRIOV
     might break.  

  
  [Original Description]
  Cavium engineers are testing  SR-IOV feature with  Cavium's new liquidIO card (based on OCTEON processor running as an intelligent NIC) on x86 Dell servers and ran into the issue described below.

  They are *not* seeing this issue on  Intel 82599 NIC.

  So far, they have seen this on multiple Dell servers - T5810  & Dell
  T630 (x86 - Ubuntu certified).

  We are testing a Cavium OCTEON device in EP mode with SRIOV.  With a
  CentOS7 VM running on the Ubuntu 14.04.2 host (using VFIO) we see that
  a single PCI read or write transaction targeting the device’s BAR0
  issued from the VM appears twice on the PCIe bus. The same accesses
  work fine when the VF is accessed directly from the Ubuntu 14.04.2
  host. These BAR0 PCI accesses do not require a driver on the VM side.
  We can reproduce the problem using a simple user-space application to
  access the VF’s BAR0 registers.

  We do not see this problem when the VM runs within a CentOS 7 host or
  under a Ubuntu 12.04 host. This appears specific to Ubuntu 14.04
  release. Appreciate your help in any clues or pointers to this
  behavior.

  This issue is also not happening with 16.04 beta.

  Steps to reproduce the bug with pcimem:

  Read:
  ./pcimem /sys/bus/pci/devices/0000\:04\:00.0/resource0 0x10080 d

  Write:
  ./pcimem /sys/bus/pci/devices/0000\:04\:00.0/resource0 0x10080 d 2048

  Read again:
  ./pcimem /sys/bus/pci/devices/0000\:04\:00.0/resource0 0x10080 d

  The value of SLIST_BAOFF_DBELL should be the same for the first read
  and the second read, after the write.

  If the bug is hit, the second read will report a value of double
  instead of the same.

  The register should have read back the same value that was written.
  The register acts like an adder in that every write adds to the
  previously written value minus anything the device has consumed. We
  see that the second read returns double the value written in the
  single write. We captured a PCIe trace and found that each of the PCI
  operation accessing this register is seen twice on the PCI bus. The 2
  writes cause the register value to double which has implications for
  normal operation. The PCIe trace is attached and has markers to
  identify the relevant transactions.

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