Re: Scalable Event Channel ABI design (draft A)

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

On Wed, 2013-02-06 at 09:38 +0000, Ian Campbell wrote:
> On Tue, 2013-02-05 at 18:02 +0000, Keir Fraser wrote:
> > On 05/02/2013 16:11, "Ian Campbell" <Ian.Campbell@citrix.com> wrote:
> > 
> > >>> Okay, I wonder how much it actually matters anyhow...
> > >>> 
> > >>> Oh by the way you say the control block is 128 bytes and will easily fit in
> > >>> the existing struct vcpu_info. That existing structure is 64 bytes in total.
> > >>> So how does that work then?
> > >> 
> > >> I meant struct vcpu_info can be extended without it growing to more than
> > >> a page.  i.e., it fits into the guest page provided in the
> > >> VCPUOP_register_vcpu_info call so no additional pages need to be
> > >> globally mapped for the control block.
> > > 
> > > VCPUOP_register_vcpu_info doesn't require each vcpu_info to be on a page
> > > by itself, even if that happens to be the Linux implementation today (I
> > > haven't checked that).
> > 
> > Having guest agree that vcpu_info grows by size of the per-vcpu control
> > block, if using this new event-channel interface, is reasonable though.
> 
> Can only use this trick once though, so it might be blocking ourselves
> into a future ABI corner.
> 

In practice embedding control block in vcpu_info might not be feasible
because there is a legacy array of vcpu_info in shared_info page. It is
quite easy to bloat shared_info to exceed size limit.

> Is there a downside to registering the control block separately? The
> guest can always arrange for them to be contiguous if it wants, or if we
> are worried about the number of global mappings then the hypervisor
> could require it shares a page with the vcpu_info but allow the offset
> to be specified separately.
> 

IMHO the global mapping space is the main concern. Regarding sharing
page with vcpu_info, this requires us to control the way kernel handles
its per-cpu section. But how?


Wei.

> Ian.
>