Re: [PATCH 3/3] KVM: Add documents for MSI-X MMIO API

[Sheng Yang <sheng@linux.intel.com>]

On Tuesday 25 January 2011 20:47:38 Avi Kivity wrote:
> On 01/19/2011 10:21 AM, Sheng Yang wrote:
> > >  >  We already got an guest MMIO address for that in the exit
> > >  >  information. I've created a chain of handler in qemu to handle it.
> > >  
> > >  But we already decoded the table and entry...
> > 
> > But the handler is still wrapped by vcpu_mmio_write(), as a part of MMIO.
> > So it's not quite handy to get the table and entry out.
> 
> The kernel handler can create a new kvm_run exit description.
> 
> >   Also the updater in the userspace
> > 
> > can share the most logic with ordinary userspace MMIO handler, which take
> > address as parameter. So I think we don't need to pass the decoded
> > table_id and entry to userspace.
> 
> It's mixing layers, which always leads to trouble.  For one, the user
> handler shouldn't do anything with the write since the kernel already
> wrote it into the table.  For another, if two vcpus write to the same
> entry simultaneously, you could see different ordering in the kernel and
> userspace, and get inconsistent results.

The shared logic is not about writing, but about interpret what's written. Old 
MMIO handler would write the data, then interpret it; and our new MMIO would only 
share the logic of interpretation. I think that's fair enough?

--
regards
Yang, Sheng