Re: [PATCH v2 4/8] ACPI/IORT: Support CANWBS memory access flag

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Aug 30, 2024 at 07:52:41AM +0000, Tian, Kevin wrote:

> But according to above description S2FWB cannot 100% guarantee it
> due to PCI No Snoop. 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