Hi Keir,

For disk read, new buffering mechanism works as follows:

1. Xen traps a single load instruction which performs PIO. FYI, IA64 doesn't have IN/OUT instructions and io ports are memory mapped. +2. Xen checks whether the data is already buffered. +3. If the data is buffered and not the last one, xen just returns the data to a guest. 4. Otherwise, xen make an i/o request to qemu as usual. And the guest blocks. 5. Qemu receives the i/o request and prepares the block of data that is buffered in qemu. +6. Qemu copies the block to a shared page which I add newly as "buffered_pio_page". That is, it exposes the qemu's buffering data to xen. 7. Qemu returns the single data on i/o request to xen as usual. 8. Xen resumes the guest.

The above lines beginning with + are newly added mechanism. 1,2,3 are almost repeated and transactions between xen and qemu are drastically reduced.

For disk write, it is almost the same as the above explanation. The difference is a direction of copy. And what I have to say is that qemu does nothing until the qemu's buffer becomes full. So the transaction to qemu can be deferred.

Thanks, Kouya

Keir Fraser writes: