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Re: [Xen-devel] Why are I/O rings bidirectional?

> Hello. I am reading Xen and the Art of Virtualization. I am curious as
> to why the async I/O ring buffers contain both requests and responses
> in the same ring.

Requests contain details of the IO to be done.  Responses notify the domain 
when the IO is complete and whether it was successful or not.

Requests can complete out of order, so it's necessary to have response 
messages following back to the domain to tell it when each request is done.  
If the requests were always dealt with in order it would not be necessary to 
have responses flowing back, so the ring could be unidirectional.

The Safe Hardware Access paper has more details about the IO rings work.


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