Re: [Intel IOMMU] Question about memory ordering in context/PASID entry updates

[Baolu Lu <baolu.lu@linux.intel.com>]

On 12/5/25 00:49, Dmytro Maluka wrote:
> When updating a context or PASID entry, we'd better make sure we set the
> present bit after setting up other bits, not before, right? And indeed,

Yes, you are right.

> for example, in domain_context_mapping_one() we are doing it in the
> following order:
> 
> 	context_clear_entry(context);
> 	<set other bits>
> 	context_set_present(context);
> 
> But it doesn't look like we have any compiler barriers to actually
> ensure this order? (And in case of context entries, there is even no
> WRITE_ONCE or anything like that.)

Yes, we should do that to ensure the order.

> So, are we just hoping that the compiler will not reorder it?
> 
> What am I missing?

We didn't implement that primarily because we hadn't encountered any
real issues, operating under the assumption that the root/context
entries population is completed before DMA translation is really
enabled.

However, that assumption is incorrect, as users can change a device's
default domain in VT-d legacy mode. This action triggers a context entry
change while DMA translation is already running. So, yes, this is a bug.

Thanks,
baolu