[Bug 1563375] [NEW] 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

[Launchpad Bug Tracker]

You have been subscribed to a private bug by Ryan Harper (raharper):

[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.

** Affects: linux (Ubuntu)
     Importance: Medium
     Assignee: Joseph Salisbury (jsalisbury)
         Status: Incomplete

** Affects: qemu (Ubuntu)
     Importance: Medium
         Status: Confirmed


** Tags: kernel-da-key trusty wily
-- 
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
https://bugs.launchpad.net/bugs/1563375
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