Re: Userspace hypercalls?

[Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

On Mon, 2007-08-27 at 20:36 +0300, Avi Kivity wrote:
> Anthony Liguori wrote:
> > On Mon, 2007-08-27 at 19:47 +0300, Avi Kivity wrote:
> >   
> >> Avi Kivity wrote:
> >>     
> >>> Thinking a little more about this, it isn't about handling hypercalls 
> >>> in userspace, but about handling a virtio sync() in userspace.
> >>>
> >>> So how about having a KVM_HC_WAKE_CHANNEL hypercall (similar to Xen's 
> >>> event channel, but assymetric) that has a channel parameter.  The 
> >>> kernel handler for that hypercall dispatches calls to either a kernel 
> >>> handler or a userspace handler.  That means we don't need a separate 
> >>> ETH_SEND, ETH_RECEIVE, or BLOCK_SEND hypercalls.
> >>>       
> >> And thinking a tiny little bit more about this, we can have the kernel 
> >> (optionally) fire an eventfd, so a separate userspace thread or process 
> >> can be woken up to service the device, without a heavyweight exit.
> >>     
> >
> >
> > Yes, I think this is much nicer.  By "calls to ... a userspace handler"
> > I presume you mean generating an exit to userspace with a new exit type
> > similar to how hypercalls work today?
> >
> >   
> 
> There are two options:
> - hypercall handler sets some fields in vcpu->run and exits to userspace
> - hypercall handler triggers an eventfd and returns to guest
> 
> Maybe we can unify the two by only allowing eventfd;

Yes, that would be better except that the latency may be unacceptable.

>  userspace can 
> attach a signal to the eventfd if it wants a synchronous exit (does 
> eventfd allow fcntl(F_SETOWN)?)

Which would address the latency issue nicely.  Looking at the fs code,
it looks like eventfd shouldn't have to do anything special for it.

Regards,

Anthony Liguori

> 


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