how emulated disk IO translated to physical disk IO on host side

how emulated disk IO translated to physical disk IO on host side

[sheng qiu]

Hi all,

i am trying to figure out the code path which translate the emulated
disk IO issued by VM to actual physical disk IO on host side. Can
anyone give me a clear view about this?

i read the kvm side code about the VMexit handling, the handle_io()
will be called for IO exits. for IO job that cannot be handled inside
the hyperviser, it will switch to qemu-kvm process and the handle_io()
at qemu side will be called. finally it seems invoke the
ioport_read()/ioport_write() which invoke the actual registered
read/write operator. Then i get lost here, i do not know for emulated
disk io which function will be responsible for the remaining job. i.e.
catch the cmd for accessing the virtual disk and translate to the
read/write to offset of the disk img file (assume we use file for
virtual disk), and then issue the system call to the host to issue the
real io cmd to physical disk.

i really hope someone can help me with it. if my understanding is not
correct, please figure out. i will be appreciated.

Thanks,
Sheng

Re: how emulated disk IO translated to physical disk IO on host side

[Stefan Hajnoczi]

On Tue, May 07, 2013 at 09:58:52AM -0500, sheng qiu wrote:
> i am trying to figure out the code path which translate the emulated
> disk IO issued by VM to actual physical disk IO on host side. Can
> anyone give me a clear view about this?

For an overview of the stack:
http://events.linuxfoundation.org/slides/2011/linuxcon-japan/lcj2011_hajnoczi.pdf

> i read the kvm side code about the VMexit handling, the handle_io()
> will be called for IO exits. for IO job that cannot be handled inside
> the hyperviser, it will switch to qemu-kvm process and the handle_io()
> at qemu side will be called. finally it seems invoke the
> ioport_read()/ioport_write() which invoke the actual registered
> read/write operator. Then i get lost here, i do not know for emulated
> disk io which function will be responsible for the remaining job. i.e.
> catch the cmd for accessing the virtual disk and translate to the
> read/write to offset of the disk img file (assume we use file for
> virtual disk), and then issue the system call to the host to issue the
> real io cmd to physical disk.

If you are running with a virtio-blk PCI adapter, then QEMU's
virtio_queue_host_notifier_read() or virtio_ioport_write() is invoked
(it depends whether ioeventfd is being used or regular I/O dispatch).

Then QEMU's hw/block/virtio-blk.c will call
bdrv_aio_readv()/bdrv_aio_writev()/bdrv_aio_flush().  This enters the
QEMU block layer (see block.c and block/) where image file formats are
handled.

Eventually you get to block/raw-posix.c which issues either a
preadv()/pwritev()/fdatasync() in a worker thread or a Linux AIO
io_submit() (if -drive aio=native was used).

Stefan