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Re: Implement mmap for PUD
OK, I've re-read this topic. Your goal is to implement blktap on
According to Xen wiki , blktap provides an interface for userland
to handle block device I/O. I guess blktap gets inter-domain, shared
ring memory from hypervisor. Dom0 userland mmaps the ring memory and
handles I/O requests.
pud(4) is different; it pretends a device driver backed by a real H/W.
Kernel passes buffers to pud(4) so that pud(4) can read/write data
from/to a real H/W, either PIO or DMA. PIO uses kernel address space
to access passed buffers. DMA uses physical addresses.
Here you want to mmap those buffers to userland, right? I don't think
it's possible. Underlying pages of given buffers are marked "busy
doing I/O" (PG_BUSY). Users (either vnode/anon owners) are not
allowed to map those pages until I/O completes. If some pud(4)
backend driver process suddenly tries to mmap those pages, VM will
surely get upset.
So a possible blktap(4) would look like:
- run only in Xen Dom0
- allocate (map) shared ring memory (both I/O requests and buffers)
from hypervisor using Xen machine-dependent API
- convert blktap I/O requests to NetBSD's bdev/strategy/struct buf format
- and call pud(4) in kernel
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