[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
Jon Grimm
jon.grimm at canonical.com
Thu Oct 6 20:14:07 UTC 2016
*** This bug is a duplicate of bug 1606940 ***
https://bugs.launchpad.net/bugs/1606940
Note, the fix for this is in trusty-proposed. It needs someone to
verify it. See:
https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1606940/comments/5
For instructions on how to provide feedback on verification. Thanks
** This bug has been marked a duplicate of bug 1606940
A a single PCI read or write appears twice on the PCIe bus. This happens when using the SR-IOV feature with some PCI devices
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https://bugs.launchpad.net/bugs/1563375
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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