using bit-field to define a qom register

[Hiroko Shimizu]

Hello,
I would like to access a 4byte-register in 1bit unit.
So, I was supposed to use a bit field to define a register like this.
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typedef struct register{
    // define register B
    uint32_t B1 : 1;
    uint32_t B2 : 1;
    uint32_t B3 : 30;
}register;
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Then, How do you define this in VMStateDescription?
And can I set a value to register.B2 from the ***_write function?

Best regards,
Hiroko