but it seems to be a SAMD21 so it should work directly

Seeeduino Xiao Board uses arduino framework and has the same µcontroler as arduino SAMD21 boards. So indeed, when I try to upload the arduino mkr zero luos led example into the Seeeduino Xiao board, it works well ! But, even if the MCU is the same for this 2 boards, they have different pinout/sercom configuration. For example, some of SAMD21 IO pins are not routed to the Seeeduino pinout. In order to solve this pb, I try to create a kind of "Luos adapted Seeeduino SAMD board", by using seeeduino variants boards, from the package "framework-arduino-samd-seeed". So, for now, I have just added the Seeeduino Xiao board config in LuosHAL folder (ATSAMD21ARDUINO -> boardconfig.h). But after compilation, I realise that there are so many things to change in order to make this board Luos compatible. So basically, I don't know what config I have to add/change to create a Luos adapted Seeeduino boards, such as Arduino SAMD boards.

Can you share the board config that you make Maybe I will find the pb?
In any case we order the board and in the next day after the shipping we should be able to provide a config to use Luos plug and play on this board

Here is the board config I made :

I could not find in pio package a variant for seeduino I'm note sure SEEEDXIAOM0 is realy taked care!
does you project compile when add Luos?

When I have added "seeedxiao.json" file to the boards folder and modified "platformio.ini", vs code automatically downloaded the package from "https://github.com/Seeed-Studio/ArduinoCore-samd", into the folder "framework-arduino-samd-seeed". So, in "luoshalconfig.h", when I check some pin definitions, it all refer to the pin definitions located on "framework-arduino-samd-seeed->variants->XIAOm0->variant.h".

ok does the sercom4_handler is register in the this variant?

I think yes because it is physically configured as uart port (according to the pinout schematic)

why don't you use plateformio default board.json choose when in plateformio.ini you have
[env:seeedxiao] platform = atmelsam board = seeedxiao
framework = arduino

yes, this is what i put on the platformio.ini file