Messages in this thread | | | Subject | Re: [PATCH v5 09/14] PCI: portdrv: Suppress kernel DMA ownership auto-claiming | From | Lu Baolu <> | Date | Fri, 7 Jan 2022 09:53:54 +0800 |
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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
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