SPI Slave multi-byte transfers

Hello,

I am trying to receive a 9 byte length SPI message with standard Altera SPI slave component in QSys. I need to use interrupts, since I can't poll for packets in the main cycle. All 9 bytes are transferred by the master during one chipselect toggle. If I'm sending only one byte, then everything work fine.

Here's my interrupt routine:

void SPI_ISR(void * context) {
    DISABLE_INTERRUPTS();
    
    IOWR_ALTERA_AVALON_SPI_STATUS(SPI_BASE, 0x00); // Clear status register
    spi_rx_value = IORD_ALTERA_AVALON_SPI_RXDATA(SPI_BASE);
    
    for(spi_index=1; spi_index<9; spi_index++) {
        while((IORD_ALTERA_AVALON_SPI_STATUS(SPI_BASE) & ALTERA_AVALON_SPI_STATUS_RRDY_MSK) == 0);
            spi_rx_value = IORD_ALTERA_AVALON_SPI_RXDATA(SPI_BASE);
    }
    ENABLE_INTERRUPTS();
}

As You can see, I get interrupt on the very first byte and poll other 8 bytes of data.

The problem:

I constantly get program stuff in a while() cycle waiting for the byte to be received. It gets stuck in random place: sometimes I see spi_index=5, sometimes spi_index=1. If I do data retransfer, then this routine gets completed and a byte from the middle of the second message triggers another interrupt. This causes complete mess.

What I am doing wrong here?

Thank You.