vfio/pci: Virtualize Maximum Payload Size

With virtual PCI-Express chipsets, we now see userspace/guest drivers
trying to match the physical MPS setting to a virtual downstream port.
Of course a lone physical device surrounded by virtual interconnects
cannot make a correct decision for a proper MPS setting.  Instead,
let's virtualize the MPS control register so that writes through to
hardware are disallowed.  Userspace drivers like QEMU assume they can
write anything to the device and we'll filter out anything dangerous.
Since mismatched MPS can lead to AER and other faults, let's add it
to the kernel side rather than relying on userspace virtualization to
handle it.
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>

drivers/vfio/pci/vfio_pci_config.c

@@ -849,11 +849,13 @@ static int __init init_pci_cap_exp_perm(struct perm_bits *perm)
 
 	/*
 	 * Allow writes to device control fields, except devctl_phantom,
-	 * which could confuse IOMMU, and the ARI bit in devctl2, which
+	 * which could confuse IOMMU, MPS, which can break communication
+	 * with other physical devices, and the ARI bit in devctl2, which
 	 * is set at probe time.  FLR gets virtualized via our writefn.
 	 */
 	p_setw(perm, PCI_EXP_DEVCTL,
-	       PCI_EXP_DEVCTL_BCR_FLR, ~PCI_EXP_DEVCTL_PHANTOM);
+	       PCI_EXP_DEVCTL_BCR_FLR | PCI_EXP_DEVCTL_PAYLOAD,
+	       ~PCI_EXP_DEVCTL_PHANTOM);
 	p_setw(perm, PCI_EXP_DEVCTL2, NO_VIRT, ~PCI_EXP_DEVCTL2_ARI);
 	return 0;
 }