Re: [PATCH RFC server 05/11] vfio-user: run vfio-user context

[John Levon]

On Fri, Aug 13, 2021 at 02:51:53PM +0000, Jag Raman wrote:

> Thanks for the information about the Blocking and Non-Blocking mode.
> 
> I’d like to explain why we are using a separate thread presently and
> check with you if it’s possible to poll on multiple vfu contexts at the
> same time (similar to select/poll for fds).
> 
> Concerning my understanding on how devices are executed in QEMU,
> QEMU initializes the device instance - where the device registers
> callbacks for BAR and config space accesses. The device is then
> subsequently driven by these callbacks - whenever the vcpu thread tries
> to access the BAR addresses or places a config space access to the PCI
> bus, the vcpu exits to QEMU which handles these accesses. As such, the
> device is driven by the vcpu thread. Since there are no vcpu threads in the
> remote process, we created a separate thread as a replacement. As you
> can see already, this thread blocks on vfu_run_ctx() which I believe polls
> on the socket for messages from client.
> 
> If there is a way to run multiple vfu contexts at the same time, that would
> help with conserving threads on the host CPU. For example, if there’s a
> way to add vfu contexts to a list of contexts that expect messages from
> client, that could be a good idea. Alternatively, this QEMU server could
> also implement a similar mechanism to group all non-blocking vfu
> contexts to just a single thread, instead of having separate threads for
> each context.

You can use vfu_get_poll_fd() to retrieve the underlying socket fd (simplest
would be to do this after vfu_attach_ctx(), but that might depend), then poll on
the fd set, doing vfu_run_ctx() when the fd is ready. An async hangup on the
socket would show up as ENOTCONN, in which case you'd remove the fd from the
set.

Note that we're not completely async yet (e.g. the actual socket read/writes are
synchronous). In practice that's not typically an issue but it could be if you
wanted to support multiple VMs from a single server, etc.


regards
john