On Thu, Jan 06, 2022 at 12:12:35PM +0800, Lu Baolu wrote: > On 1/5/22 1:06 AM, Bjorn Helgaas wrote: > > On Tue, Jan 04, 2022 at 09:56:39AM +0800, Lu Baolu wrote: > > > If a switch lacks ACS P2P Request Redirect, a device below the switch can > > > bypass the IOMMU and DMA directly to other devices below the switch, so > > > all the downstream devices must be in the same IOMMU group as the switch > > > itself. > > Help me think through what's going on here. IIUC, we put devices in > > the same IOMMU group when they can interfere with each other in any > > way (DMA, config access, etc). > > > > (We said "DMA" above, but I guess this would also apply to config > > requests, right?) > > I am not sure whether devices could interfere each other through config > space access. The IOMMU hardware only protects and isolates DMA > accesses, so that userspace could control DMA directly. The config > accesses will always be intercepted by VFIO. Hence, I don't see a > problem. I was wondering about config accesses generated by an endpoint, e.g., an endpoint doing config writes to a peer or the upstream bridge. But I think that is prohibited by spec - PCIe r5.0, sec 7.3.3, says "Propagation of Configuration Requests from Downstream to Upstream as well as peer-to-peer are not supported" and "Configuration Requests are initiated only by the Host Bridge, including those passed through the SFI CAM mechanism." Bjorn
