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Does it suggest that we should only allow nesting > only for CANWBS, or disable/hide PCI No Snoop cap from the guest > in case of S2FWB? ARM has always had an issue with no-snoop and VFIO. The ARM expectation is that VFIO/VMM would block no-snoop in the PCI config space. >From a VM perspective, any VMM on ARM has to take care to do this today already. For instance a VMM could choose to only assign devices which never use no-snoop, which describes almost all of what people actually do :) The purpose of S2FWB is to keep that approach working. If the VMM has blocked no-snoop then S2FWB ensures that the VM can't use IOPTE bits to break cachability and it remains safe. >From a VFIO perspective ARM has always had a security hole similer to what Yan is trying to fix on Intel, that is a separate pre-existing topic. Ideally the VFIO kernel would block PCI config space no-snoop for alot of cases. Jason