On Tue, Feb 18, 2025 at 05:24:08AM +0000, Tian, Kevin wrote:

quoted

From: Nicolin Chen <redacted>
Sent: Saturday, January 25, 2025 8:31 AM

There is a DoS concern on the shared hardware event queue among devices
passed through to VMs, that too many translation failures that belong to
VMs could overflow the shared hardware event queue if those VMs or their
VMMs don't handle/recover the devices properly.

This statement is not specific to the nested configuration.

quoted

The MEV bit in the STE allows to configure the SMMU HW to merge similar
event records, though there is no guarantee. Set it in a nested STE for
DoS mitigations.

Is MEV available only in nested mode? Otherwise it perhaps makes
sense to turn it on in all configurations in IOMMUFD paths...

MEV is available at all times (if an implemented by the HW) and doesn't
depend on the nested mode. As per the Arm SMMUv3 spec (section 3.5.5):

Events can be merged where all of the following conditions are upheld:
 - The event types and all fields are identical, except fields explicitly
indicated in section 7.3 Event records.

 - If present, the Stall field is 0. Stall fault records are not merged.

I'm not sure to what extent, but I think *trying* to merge similar event
should reduce some chances of overflowing the hw eventq.

Is MEV available only in nested mode? Otherwise it perhaps makes
sense to turn it on in all configurations in IOMMUFD paths...

I think the arm-smmu-v3's iommufd implementation only supports nested
which could be the reason.

Thanks,
Praan