On Sat, Dec 09, 2017 at 04:23:17PM +0000, Wang, Wei W wrote:
On Friday, December 8, 2017 4:34 PM, Stefan Hajnoczi wrote:
On Fri, Dec 8, 2017 at 6:43 AM, Wei Wang <address@hidden>
wrote:
On 12/08/2017 07:54 AM, Michael S. Tsirkin wrote:
On Thu, Dec 07, 2017 at 06:28:19PM +0000, Stefan Hajnoczi wrote:
On Thu, Dec 7, 2017 at 5:38 PM, Michael S. Tsirkin
<address@hidden>
Thanks Stefan and Michael for the sharing and discussion. I
think above 3 and 4 are debatable (e.g. whether it is simpler
really depends). 1 and 2 are implementations, I think both
approaches could implement the device that way. We originally
thought about one device and driver to support all types (called
it transformer sometimes :-) ), that would look interesting from
research point of view, but from real usage point of view, I
think it would be better to have them separated,
because:
- different device types have different driver logic, mixing
them together would cause the driver to look messy. Imagine that
a networking driver developer has to go over the block related
code to debug, that also increases the difficulty.
I'm not sure I understand where things get messy because:
1. The vhost-pci device implementation in QEMU relays messages but
has no device logic, so device-specific messages like
VHOST_USER_NET_SET_MTU are trivial at this layer.
2. vhost-user slaves only handle certain vhost-user protocol messages.
They handle device-specific messages for their device type only.
This is like vhost drivers today where the ioctl() function
returns an error if the ioctl is not supported by the device. It's not messy.
Where are you worried about messy driver logic?
Probably I didn’t explain well, please let me summarize my thought a
little
bit, from the perspective of the control path and data path.
Control path: the vhost-user messages - I would prefer just have the
interaction between QEMUs, instead of relaying to the GuestSlave,
because
1) I think the claimed advantage (easier to debug and develop)
doesn’t seem very convincing
You are defining a mapping from the vhost-user protocol to a custom
virtio device interface. Every time the vhost-user protocol (feature
bits, messages,
etc) is extended it will be necessary to map this new extension to the
virtio device interface.
That's non-trivial. Mistakes are possible when designing the mapping.
Using the vhost-user protocol as the device interface minimizes the
effort and risk of mistakes because most messages are relayed 1:1.
2) some messages can be directly answered by QemuSlave , and some
messages are not useful to give to the GuestSlave (inside the VM),
e.g. fds, VhostUserMemoryRegion from SET_MEM_TABLE msg (the device
first maps the master memory and gives the offset (in terms of the
bar, i.e., where does it sit in the bar) of the mapped gpa to the
guest. if we give the raw VhostUserMemoryRegion to the guest, that wouldn’t be
usable).
I agree that QEMU has to handle some of messages, but it should still
relay all (possibly modified) messages to the guest.
The point of using the vhost-user protocol is not just to use a
familiar binary encoding, it's to match the semantics of vhost-user
100%. That way the vhost-user software stack can work either in host
userspace or with vhost-pci without significant changes.
Using the vhost-user protocol as the device interface doesn't seem any
harder than defining a completely new virtio device interface. It has
the advantages that I've pointed out:
1. Simple 1:1 mapping for most that is easy to maintain as the
vhost-user protocol grows.
2. Compatible with vhost-user so slaves can run in host userspace
or the guest.
I don't see why it makes sense to define new device interfaces for
each device type and create a software stack that is incompatible with
vhost-user.