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

[Avi Kivity <avi@redhat.com>]

On 01/26/2011 11:05 AM, Sheng Yang wrote:
> 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?

It dosn't make sense for an API point of view.  You registered a table 
of entries, you expect an exit on that table to point to the table and 
entry that got changed.

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