Why guest is disallowed to change mask bit

[Chien-Hua Yen <chien.yen@oracle.com>]

Hi,

I am curious to know why Xen disallows guest to change the mask bit of 
MSI-X
vector control as show in the comment out section in msixtbl_write().
Our SR-IOV driver got driver reload failure because it cannot enable 
interrupt.

      /* Do not allow the mask bit to be changed. */
#if 0 /* XXX
        * As the mask bit is the only defined bit in the word, and as the
        * host MSI-X code doesn't preserve the other bits anyway, doing
        * this is pointless. So for now just discard the write (also
        * saving us from having to determine the matching irq_desc).
        */
     spin_lock_irqsave(&desc->lock, flags);
     orig = readl(virt);
     val &= ~PCI_MSIX_VECTOR_BITMASK;
     val |= orig & PCI_MSIX_VECTOR_BITMASK;
     writel(val, virt);
     spin_unlock_irqrestore(&desc->lock, flags);
#endif

     r = X86EMUL_OKAY;

Thanks.

Chien