On Sat, Apr 19, 2014 at 12:08:15PM +0800, Amos Kong wrote:

Hi all,

I'm working on this item in Upstream Networking todolist:

| - Sharing config interrupts
|    Support more devices by sharing a single msi vector
|    between multiple virtio devices.
|    (Applies to virtio-blk too).

I have this solution here, and only have draft patches of Solution
1 & 2, let's discuss if solution 3 is feasible.

* We should not introduce perf regression in this change
* little effect is acceptable because we are _sharing_ interrupt

Solution 1:
==========
share one LSI interrupt for configuration change of all virtio devices.
Draft patch: share_lsi_for_all_config.patch

Solution 2:
==========
share one MSI interrupt for configuration change of all virtio devices.
Draft patch: share_msix_for_all_config.patch

Solution 3:
==========
dynamic adjust interrupt policy when device is added or removed
Current:
-------
- try to allocate a MSI to device's config
- try to allocate a MSI for each vq
- share one MSI for all vqs of one device
- share one LSI for config & vqs of one device

additional dynamic policies:
---------------------------
- if there is no enough MSI, adjust interrupt allocation for returning
  some MSI
   - try to find a device that allocated one MSI for each vq, change
     it to share one MSI for saving the MSI
   - try to share one MSI for config of all virtio_pci devices
   - try to share one LSI for config of all devices

- if device is removed, try to re-allocate freed MSI to existed
  devices, if they aren't in best status (one MSI for config, one
  MSI for each vq)
   - if config of all devices is sharing one LSI, try to upgrade it to MSI
   - if config of all devices is sharing one MSI, try to allocate one
     MSI for configuration change of each device
   - try to find a device that is sharing one MSI for all vqs, try to
     allocate one MSI for each vq



BTW, I saw we still notify all vqs even VIRTIO_PCI_ISR_CONFIG bit of
isr is set, is it necessary?

[Reply myself]

Quote from Virtio-spec:

| 2.4.3 Dealing With Configuration Changes
| Some virtio PCI devices can change the device configuration state, as reflected
| in the virtio header in the PCI configuration space. In this case:
| 
| 1. If MSI-X capability is disabled: an interrupt is delivered and the sec-
| ond highest bit is set in the ISR Status field to indicate that the driver
| should re-examine the configuration space.

| Note that a single interrupt can
| indicate both that one or more virtqueue has been used and that the con-
| figuration space has changed: even if the config bit is set, virtqueues must
| be scanned. <<<<

It seems current code is fine.