wchTest_UART_int.ino

const size_t usart1_buff_size = 256;
char usart1_buff[usart1_buff_size];
uint16_t usart1_buff_write = 0;
uint16_t usart1_buff_read = 0;

#ifdef __cplusplus
extern "C" {
#endif
  void USART1_IRQHandler(void) __attribute__((interrupt("WCH-Interrupt-fast")));
  void USART1_IRQHandler(void) {
    USART_ClearITPendingBit(USART1, USART_IT_RXNE);
    usart1_buff[usart1_buff_write] = USART_ReceiveData(USART1);
    usart1_buff_write++;
    usart1_buff_write %= usart1_buff_size;
  }
#ifdef __cplusplus
}
#endif

int usart1_available() {
  return (usart1_buff_write != usart1_buff_read);
}

char usart1_get() {
  uint8_t c;
  if (usart1_buff_write != usart1_buff_read) {
    c = usart1_buff[usart1_buff_read];
    usart1_buff_read++;
    usart1_buff_read %= usart1_buff_size;
    return c;
  }
  return 0;
}


void setup() {
  Serial.begin(115200);
  USART_ITConfig(USART1, USART_IT_RXNE, ENABLE);
  NVIC_SetPriority(USART1_IRQn, UART_IRQ_PRIO);
  NVIC_EnableIRQ(USART1_IRQn);
}

void loop() {
  while (usart1_available()) {
    Serial.print(usart1_get());
  }

  delay(1000);
}

Excellent code! I'm currently using this to to see how I can integrate using interrupts in the CH32 Arduino core.
I'm testing it on the CH32V003 to improve my clumsy polling implementation.

https://github.com/ch32-riscv-ug/arduino_core_ch32_riscv_arduino

I am making a new Arduino Core.
It is not yet complete.
One of its features is that the EVT works as is.

Impressive! That's quite a bit of work you've taken upon yourself.
I'm currently using the WCH Arduino core for my CH32V003 projects. Using the Arduino API allows me to reuse many libraries.
After getting features such as working I2C and Serial working, I now often run into the limited flash space, partly caused by HAL overhead. Your core promises a smaller footprint. Good to see potential alternatives coming up...

For the WCH core I've contributed features such as I2C slave and Eeprom emulation. Your code helped me with interrupt driven Serial RX. Thank you! (I'll probably submit my PR for those changes sometime soon.)