On 1/7/22 2:32 AM, Bjorn Helgaas wrote:
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."
That's clear. Thank you for the clarification.
Bjorn
Best regards,
baolu
