Hi all:
To get native performance of VF, we need to map doorbell to guest to
avoid unnecessary vmexit. In order to do this, we will launch qemu with
page-per-vq=on. This means the each doorbell register should be located
at the beginning of 4K page and does not share the page with other
registers. Then vDPA framework can safely map it into the guest physical
address (GPA) range defined by qemu. It could be either
1) a single doorbell register that is used by all virtqueues
or
2) several different per-vq doorbell registers
If you decide to implement a virtio-pci register layout, need to make
sure for notification structure (4.1.4.4 of virtio spec):
For each virtqueue, the result
ofcap.offset+queue_notify_off*notify_off_multiplier is PAGE_SIZE (e.g
4K) alignment, and the doorbeel does not share the page with other
registers.
And it would be better if queue_notify_off, notify_off_multiplier can be
changed via firmware for extra flexibility.
Please check and make sure your hardware have such ability and feel free
to ask if you have questions (offline if necessary).
Thanks
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