On Wed, Dec 13, 2017 at 10:59:10PM +0200, Michael S. Tsirkin wrote: > > * VHOST_USER_SET_VRING_KICK > > > > Set up vring kick doorbell (unless bit 8 is set) before sending > > VHOST_USER_SET_VRING_KICK to the guest. > > But guest can't use it, now can it? > > What guest needs is a mapping to interrupts. ... > > * VHOST_USER_SET_VRING_CALL > > > > Set up the vring call doorbell (unless bit 8 is set) before sending > > VHOST_USER_SET_VRING_CALL to the guest. > > Same here. what guest needs is mapping from io to notifications, > right?
The PCI device should contain a BAR with doorbell registers. I don't think a fancy mapping is necessary, instead the device spec should define the BAR layout. When the guest vhost-user slave receives this message it knows it can now begin using the doorbell register. > --- > > > > I took a quick look and I doubt we can do something that is both > > > compatible with the existing vhost-user and will make it possible to > > > extend the protocol without qemu changes. Let's assume I pass a new > > > message over the vhost-user channel. How do we know it's safe to pass > > > it to the guest? > > > > > > That's why we gate any protocol change on a feature bit and must parse > > > all messages. > > > > QEMU must parse all messages and cannot pass through unknown messages. > > Think of QEMU vhost-pci as both a vhost-user slave to the other VM and > > a vhost-user master to the guest. > > > > QEMU changes are necessary when the vhost protocol is extended. > > Device interface changes are only necessary if doorbells or shared > > memory regions are added, any other protocol changes do not change the > > device interface. > > > > Stefan > > I guess you have a different definition of a device interface than > myself - I consider it an interface change if a feature bit changes :) The feature bits are defined in the vhost-user protocol specification, not in the vhost-pci PCI device specification. For example, imagine we are adding the virtio-net MTU feature to the protocol: 1. The VHOST_USER_PROTOCOL_F_MTU feature bit is added to the vhost-user protocol specification. 2. The VHOST_USER_NET_SET_MTU message is added to the vhost-user protocol specification. As a result of this: 1. No PCI adapter resources (BARs, register layout, etc) change. This is why I say the device interface is unchanged. The vhost-pci specification does not change. 2. QEMU vhost-pci code needs to unmask VHOST_USER_PROTOCOL_F_MTU and pass through VHOST_USER_NET_SET_MTU. Stefan
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