Re: [patch 1/2] irqchip/msi-lib: Honor the MSI_FLAG_PCI_MSI_MASK_PARENT flag

[Thomas Gleixner]

On Mon, Dec 22 2025 at 16:16, Marc Zyngier wrote:
>> > > Once we remove the costly readback, is there any remaining reason
>> > > to overwrite [un]mask_irq() with irq_chip_[un]mask_parent() ?
>> >
>> > So you are effectively not masking at all and just rely on hope
>> > instead. I have the utmost confidence in this sort of stuff. Totally.
>> 
>> I don't understand the above comment.
>> Masking happens as a result of the PCIe write,
>> which will eventually reach the device. The presence of the
>> readback does nothing to accelerate the landing of the write.
>
> It doesn't accelerate it. It *guarantees* that the write is observed
> and has taken effect. It acts as a completion barrier. Without it, the
> write can be buffered at an arbitrary location in the interconnect, or
> stored in the device but not acted upon.
>
> What you have here is the equivalent of throwing a message in a bottle
> at sea, and expecting a guaranteed reply.

  https://xkcd.com/3150/