[Bug 202055] Failed to PCI passthrough SSD with SMI SM2262 controller.

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=202055

--- Comment #42 from Alex Williamson (alex.williamson@redhat.com) ---
It seems there's a partial workaround available since QEMU v2.12 hiding under
our noses.  That version adds support for relocating the MSI-X vector table on
vfio-pci devices, which recreates the MSI-X MMIO space elsewhere on the device.
 A side-effect of this is that the vector table and PBA are properly sized so
as not to collide.  The size of the tables remains wrong, but this only becomes
a problem if the nvme code attempts to allocate >16 vectors, which requires >15
vCPU (or host CPUs if the device is returned to host drivers after being
assigned)(nvme appears to allocate 1 admin queue, plus a queue per CPU, each
making use of an IRQ vector).  The QEMU vfio-pci device option is
x-msix-relocation= which allows specifying the bar to use for the MSI-X tables,
ex. bar0...bar5.  Since this device uses a 64bit bar0, we can either extend
that BAR or choose another, excluding bar1, which is consumed by the upper half
of bar0.  For instance, I tested with:

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
...
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
      </source>
      <alias name='ua-sm2262'/>
      <address type='pci' domain='0x0000' bus='0x02' slot='0x00'
function='0x0'/>
    </hostdev>
...
  <qemu:commandline>
    <qemu:arg value='-set'/>
    <qemu:arg value='device.ua-sm2262.x-msix-relocation=bar2'/>
  </qemu:commandline>


(NB: "ua-" is a required prefix when specifying an alias)

A new virtual BAR appears in the guest hosting the MSI-X table and QEMU starts
normally so long as the guest doesn't exceed 15 vCPUs.

The vCPU/pCPU count limitations are obviously not ideal, but hopefully this
provides some degree of workaround for typical configurations.

-- 
You are receiving this mail because:
You are watching the assignee of the bug.