Re: [PATCH] device-tree: optimize dt_device_for_passthrough()

[Julien Grall <julien@xxxxxxx>]

On 11/02/2025 11:57, Orzel, Michal wrote:
> On 11/02/2025 12:18, Grygorii Strashko wrote:
> > The dt_device_for_passthrough() is called many times during Xen
> > initialization and Dom0 creation. On every call it traverses struct
> > dt_device_node properties list and compares compares properties name with
> double "compares"
>
> > "xen,passthrough" which is runtime overhead. This can be optimized by
> Are you sure? Looking at the calls, it's almost only used at boot except for dt
> overlay.
>
> > marking dt_device_node as passthrough while unflattening DT.
> >
> > This patch introduced new struct dt_device_node property "is_passthrough"
> > which is filled if "xen,passthrough" property is present while unflattening
> > DT and dt_device_for_passthrough() just return it's value.
> In the past we were skeptical about adding new fields to the dt_device_node
> structure for use cases like this one. I would say this optimization is not
> worth it. Also, why would you optimize dt_device_for_passthrough but not
> e.g. dt_device_is_available.
So we are trading speed with memory usage. It looks like we may be using 
a padding, although I didn't check whether the existing structure could 
be packed...
> You can check with other Arm maintainers.
Before forging an opinion, I'd like to see some numbers showing the 
performance improvement. Also, is this impacting only boot?
Also, I agree with Michal, if this is a concern for 
dt_device_device_for_passthrough(). Then it would be a concern for a few 
others calls using dt_find_property(). Are you going to modify all of them?
Cheers,

--
Julien Grall