Re: FIFO-based event channel ABI design (draft B)

[Wei Liu <wei.liu2@citrix.com>]

On Fri, 2013-02-15 at 18:46 +0000, David Vrabel wrote:
> On 15/02/13 18:19, Wei Liu wrote:
> > Apart from the TAIL field, I have some comments on the hypercall
> > interface. Please see inline comments.
> > 
> > On Fri, 2013-02-15 at 14:32 +0000, David Vrabel wrote:
> >>
> >> ### `EVTCHNOP_init_control`
> >>
> >> This call initializes a single VCPU's control block.
> >>
> >> A guest should call this during initial VCPU bring up.  The guest must
> >> have already successfully registered a vcpu_info structure and the
> >> control block must be in the same page.
> >>
> >> If this call fails on the boot VCPU, the guest should continue to use
> >> the 2-level event channel ABI for all VCPUs.  If this call fails on
> >> any non-boot VCPU then the VCPU will be unable to receive events and
> >> the guest should offline the VCPU.
> >>
> > 
> > Why offline this CPU? This call only registers control block, we can
> > switch back to use 2-level for all CPUs.
> 
> Because switching back for the other VCPUs is potentially impossible (we
> may have hotplugged this new VCPU and more than 4096 event channels may
> be currently in use).  It would also require migrating events between
> the new and old data structures which is hard.
> 

If you hot plug a vcpu, can it possibly bypass the cpu_possible_map
limit?

VCPU info placement call is issued for all possible CPU's, in this case,
you can have new ABI setup for all possible CPUs at start-of-day, so
there is no migration needed.

> I would expect this call to never fail in normal operation (except on
> the boot VCPU where support may be missing).  i.e., it will only fail if
> xenheap or map space is exhausted.  Both the xenheap and the map space
> should be large enough so we run out of other resources (e.g,, domheap)
> first.
> 
[snip]
> 
> The boot VCPU shall have successfully initialized its control block.  I
> don't think there are any other restrictions on this call.
> 

So do you mean that this is only called when guest starts?


Wei.