RE: [RFC v6 06/10] PCI: Add a new PCI_BUS_FLAGS_MSI_REMAP flag

[David Laight]

From: Yongji Xie
> Sent: 18 April 2016 11:59
> We introduce a new pci_bus_flags, PCI_BUS_FLAGS_MSI_REMAP
> which indicates all devices on the bus are protected by the
> hardware which supports IRQ remapping(intel naming).
> 
> This flag will be used to know whether it's safe to expose
> MSI-X tables of PCI BARs to userspace. Because the capability
> of IRQ remapping can guarantee the PCI device cannot trigger
> MSIs that correspond to interrupt IDs of other devices.

I'm worried that this entire series is going to break drivers
for existing hardware.

I understand some of the reasoning for 'vm pass through' configurations,
but there will be PCIe devices out there that have the MSI-X tables
in the same BAR as other device registers.
If you are lucky nothing else is in the same 4k area, but I wouldn't
assume it.

In any case, if the hardware can't police the card's master transfers
there is nothing to stop a different bus master block on the card
from raising MSI-X interrupts - they are just a PCIe write.
So all you are doing is raising the bar slightly and giving a very false
sense of security.

	David