Re: [Qemu-devel] Questions regarding ivshmem spec

[Sasha Levin]

On Thu, 2011-08-25 at 17:40 +0300, Avi Kivity wrote:
> On 08/25/2011 05:39 PM, Sasha Levin wrote:
> > On Thu, 2011-08-25 at 17:00 +0300, Avi Kivity wrote:
> > >  On 08/25/2011 04:29 PM, Sasha Levin wrote:
> > >  >  2. The spec describes DOORBELL as an array of DWORDs, when one guest
> > >  >  wants to poke a different guest it would write something into the 
> > > offset
> > >  >  of the other guest in the DOORBELL array.
> > >  >  Looking at the implementation in QEMU, DOORBELL is one DWORD, when
> > >  >  writing to it the upper WORD is the guest id and the lower WORD is the
> > >  >  value.
> > >  >  What am I missing here?
> > >  >
> > >
> > >  The spec in qemu.git is accurate.  The intent is to use an ioeventfd
> > >  bound into an irqfd so a write into the doorbell injects an interrupt
> > >  directly into the other guest, without going through qemu^Wkvm tool.
> > >
> >
> > But the doorbell is a single DWORD, so if a guest writes to it we'd
> > still need to figure out which guest/vector he wants to poke from
> > userspace, no?
> >
> > If it was an array of doorbells then yes, we could assign an ioeventfd
> > to each offset - but now I don't quite see how we can avoid passing
> > through the userspace.
> >
> 
> Use the datamatch facility.
> 
> We didn't want an array of registers to avoid scaling issues (PIO space 
> is quite small).
> 
> 

Ah, right.

Thanks!

-- 

Sasha.