Hi,
On 11/02/2025 15:14, Grygorii Strashko wrote:
On 11.02.25 14:32, Julien Grall wrote:
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"
10x
"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...
Actually I see it consumes nothing due to alignments:
- before sizeof(dt_device_node)=144
- after sizeof(dt_device_node)=144
But it could be made bool is_passthrough:1; together with other bools,
and probably moved at the end of struct dt_device_node.
By the way, below diff can save 8 bytes on arm64 per struct dt_device_node.
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?
Sry, indeed only boot, need to be more specific.
My DT: ~700 nodes
Number of calls till the end of create_dom0():
(XEN) =============== dt_device_for_passthrough=2684
dt_device_is_available=1429.
I've enabled console_timestamps=boot and did 5 boots and calculated
average - it gives ~20 microseconds improvement.
This doesn't seem to be worth it. But I also don't know what's the
normal boot time on your system... I guess we are still in seconds?
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?
I follow the rule one patch one functional change. Regarding further
optimization - I think only generic properties can be optimized and
personally I see now only "xen,passthrough" and may be "status".
Ok. 20 microseconds. it's probably more like a measurement error margin.
Please advice if i should continue or drop?
See above. Regardless that, would you be able to provide a bit more
information of your end goal? Are you intending to be able to boot
Xen/dom0/dom0less guest in less than N milliseconds? How far are you
from that target? Are those numbers done on the latest Xen?
Asking because there are probably bigger place where optimization can be
done first.
Cheers,
--
Julien Grall
