From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tim Deegan <tim@xen.org>
Subject: Re: [PATCH 4/5] xen: events,
	exposes evtchn_alloc_unbound_domain
Date: Thu, 9 Aug 2012 11:35:57 +0100
Message-ID: <20120809103557.GA17503@ocelot.phlegethon.org>
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To: Ian Campbell <Ian.Campbell@citrix.com>
Cc: "Jean Guyader (3P)" <jean.guyader@citrix.com>, "xen-devel@lists.xen.org" <xen-devel@lists.xen.org>
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At 11:23 +0100 on 09 Aug (1344511426), Ian Campbell wrote:
> On Thu, 2012-08-09 at 11:06 +0100, Tim Deegan wrote:
> > At 20:50 +0100 on 03 Aug (1344027053), Jean Guyader wrote:
> > > 
> > > Exposes evtchn_alloc_unbound_domain to the rest of
> > > Xen so we can create allocated unbound evtchn within Xen.
> > > 
> > > Signed-off-by: Jean Guyader <jean.guyader@citrix.com>
> > 
> > > @@ -161,18 +163,18 @@ static long evtchn_alloc_unbound(evtchn_alloc_unbound_t *alloc)
> > >  {
> > >      struct evtchn *chn;
> > >      struct domain *d;
> > > -    int            port;
> > > +    evtchn_port_t  port;
> > >      domid_t        dom = alloc->dom;
> > > -    long           rc;
> > > +    int            rc;
> > 
> > The function returns long; if you're tidying this up to be an int, might
> > as well change the return type too.
> 
> I'm not sure if this is relevant but Jan just sent a patch to "make all
> (native) hypercalls consistently have "long" return type". I
> think/suspect this rc here turns into the result of the hypercall?
> 
> Jan's patch was motivated by something to do with sign extension when a
> hypercall's int return is written to the long in the multicall arg
> struct which causes strangeness. Perhaps not totally relevant to
> evtchn_alloc which is unlikely to be in a MC.

Yes, this eventually ends up in a hypercall handler, but s/long/int/
here doesn't cause problems because
 - rc is only ever set to an 'int' value here so we can't lose data
   from the type being too narrow; and
 - Those int values get cast up to long (either in here or in the
   caller) directly, which will sign-extend the.

It really doesn't matter whether this function returns an int or a long,
but it's a bit untidy to change it half-way. 

Tim.