Re: [PATCH 2/3] xen-fbfront: Use grant references when requested

[Ian Campbell <Ian.Campbell@eu.citrix.com>]

On Fri, 2011-01-07 at 21:02 +0000, Daniel De Graaf wrote: 
> On 01/07/2011 03:44 PM, Ian Campbell wrote:
> > More importantly there is also no way for the backend to figure out is
> > the frontend is going to obey the request if it does write it (at least
> > until it tries to map a gref and fails because its really got an mfn).
> 
> The key used in xenstore (page-gref versus page-ref) tells the backend if
> the request was honored.

Oh yes, of course. I even suggested the same thing myself later on...

> > Usually both front and backend would write a feature-foo node to their
> > respective directory in xenstore and then figure out what to do based on
> > what the other end wrote.
> 
> Perhaps "use-grants" should be renamed to "feature-grants"?

I think it would be more consistent with the other protocols (and the
existing feature-update in the fb protocol) to do so. Also use-grants
sounds a bit like a command rather than a suggestion to me.

> I thought the
> frontend writing a feature-grants node would be redundant since the
> page-gref node already communicated what was chosen.

Agreed.

> > In the kbdfront patch you simply write both the mfn and the grant
> > reference to xenstore, presumably the backend then just picks for itself
> > which access method to use, could that approach be applicable here?
> 
> Possible, but it would waste a bit of memory (a few extra pages for the
> MFN tables in addition to the grant-ref tables). If this isn't an issue,
> then fbfront can expose both and allow the backend to choose without any
> negotiation. Is this preferred?

Since you've already got the negotiation aspect covered I don't think
there is any need go down this path.

Ian.