From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ian Campbell <ian.campbell@citrix.com>
Subject: Re: [PATCH XEN v5 09/23] tools: Refactor hypercall
 calling wrappers into libxencall.
Date: Fri, 13 Nov 2015 15:17:32 +0000
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To: Wei Liu <wei.liu2@citrix.com>, Ian Jackson <Ian.Jackson@eu.citrix.com>
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On Wed, 2015-11-11 at 15:31 +0000, Wei Liu wrote:
> On Wed, Nov 11, 2015 at 03:08:13PM +0000, Ian Jackson wrote:
> > Ian Campbell writes ("[PATCH XEN v5 09/23] tools: Refactor hypercall
> > calling wrappers into libxencall."):
> > > libxencall will provide a stable API and ABI for calling hypercalls
> > > (although those hypercalls themselves may not have a stable API). As
> > > well as the hypercall buffer infrastructure needed in order to safely
> > > provide pointer arguments to hypercalls.
> > ...
> > > +/*
> > > + * This library allows you to make arbitrary hypercalls (subject to
> > > + * sufficient permission for the process and the domain itself).
> > > Note
> > > + * that while the library interface is stable the hypercalls are
> > > + * subject to their own rules.
> > 
> > Something needs to say what the error handling is like.
> > 
> > Do these functions set errno ?
> > 
> > > +/*
> > > + * Call hypercalls with varying numbers of arguments.
> > > + */
> > > +int xencall0(xencall_handle *xcall, unsigned int op);
> > 
> > Is the return value the raw hypercall return value, or is hypervisor
> > do_foo returning -EFOOBAR turned into to -1/errno=EFOOBAR ?
> > (Hopefully the answer to this doesn't depend on the hypercall ABI...)
> > 
> 
> That is -1/errno=E_XEN_FOOBAR across all OSes.
> 
> (I think this stems from Linux ioctl semantics. Then NetBSD etc follow
> suit.)

I wish it were that simple :-(