On Wed, Feb 02, 2022 at 08:49:33AM -0700, Alex Williamson wrote: > > Alex, what did you refer to? > > My evidence is largely by omission, but that might be that in practice > it's not used rather than explicitly forbidden. I note that the bus > master enable bit specifies: > > Bus Master Enable - Controls the ability of a Function to issue > Memory and I/O Read/Write Requests, and the ability of > a Port to forward Memory and I/O Read/Write Requests in > the Upstream direction. > > That would suggest it's possible, but for PCI device assignment, I'm > not aware of any means through which we could support this. There is > no support in the IOMMU core for mapping I/O port space, nor could we > trap such device initiated transactions to emulate them. I can't spot > any mention of I/O port space in the VT-d spec, however the AMD-Vi spec > does include a field in the device table: > > controlIoCtl: port I/O control. Specifies whether > device-initiated port I/O space transactions are blocked, > forwarded, or translated. > > 00b=Device-initiated port I/O is not allowed. The IOMMU target > aborts the transaction if a port I/O space transaction is > received. Translation requests are target aborted. > > 01b=Device-initiated port I/O space transactions are allowed. > The IOMMU must pass port I/O accesses untranslated. Translation > requests are target aborted. > > 10b=Transactions in the port I/O space address range are > translated by the IOMMU page tables as memory transactions. > > 11b=Reserved. > > I don't see this field among the macros used by the Linux driver in > configuring these device entries, so I assume it's left to the default > value, ie. zero, blocking device initiated I/O port transactions. > > So yes, I suppose device initiated I/O port transactions are possible, > but we have no support or reason to support them, so I'm going to go > ahead and continue believing any I/O port address space from the device > perspective is largely irrelevant ;) Thanks, > > Alex
Right, it would seem devices can initiate I/O space transactions but IOMMUs don't support virtualizing them and so neither does VFIO. -- MST
