PCI Passthrough does not work for Virtual Machine with Linux kernel 4.2 and above

Products

• VMware vCenter Server
• VMware vSphere ESXi

Issue/Introduction

Symptoms:

• The PCI Passthrough device does not work in a Linux Virtual Machine with kernel version 4.2 and above
• In the vmware.log file, you see message similar to:

Enable MSIX with zero vector programmed, intr proxy will not be set up, device:xxxx:xx:xx.x

Environment

VMware vSphere ESXi 5.5
VMware vCenter Server Appliance 5.5.x
VMware vCenter Server 5.5.x

Cause

Patches in https://lkml.org/lkml/2015/4/13/21 changed the way the Linux kernel programmed MSI-X vectors.

Older versions programs the MSI-X vector table before enabling MSI-X in the message control register but newer versions do the following:

• Sets the MSI-X message control register to enabled + masked
• Masks all vectors in the MSI-X table
• Unmasks MSI-X via the message control register
• Programs the vectors in the MSI-X table and unmasks them

This breaks the assumptions made by PCI passthru code - specifically PCIPassthruSetupInterrupt is only called when the guest touches the message control register and PCIPassthruChangeIntrSettings assumes that the message addresses have already been programmed.

Resolution

To resolve this issue:

• Upgrade the affected ESXi host to 5.5 Patch 08 or later version.
• Upgrade the affected ESXi host to ESXi 6.0 Patch 03 or later version.
  
  These versions fix the problem by making PCIPassthruMSIXRPCHandler handle this case by disabling MSI-X before the guest writes the vector table and re-enabling it afterwards if MSI-X is enabled through the message control register.

Impact/Risks

ESXi host needs to be rebooted to apply the fix.