Re: [Xen-devel] [PATCH v8 4/5] libxc: Provide set_xen_guest_handle_offset macro

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 05/08/2015 11:56 AM, Ian Campbell wrote:
> On Fri, 2015-05-08 at 11:41 -0400, Boris Ostrovsky wrote:
> > On 05/08/2015 10:57 AM, Ian Campbell wrote:
> > > On Wed, 2015-05-06 at 14:15 -0400, Boris Ostrovsky wrote:
> > > > Add set_xen_guest_handle_offset() macro that can be used for setting
> > > > xen_guest_handle to an offset into hypercall buffer.
> > > >
> > > > Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > > > ---
> > > >    tools/libxc/include/xenctrl.h |   11 ++++++++---
> > > >    1 files changed, 8 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/tools/libxc/include/xenctrl.h b/tools/libxc/include/xenctrl.h
> > > > index 8c8eae6..4e6f62c 100644
> > > > --- a/tools/libxc/include/xenctrl.h
> > > > +++ b/tools/libxc/include/xenctrl.h
> > > > @@ -322,16 +322,21 @@ typedef struct xc_hypercall_buffer xc_hypercall_buffer_t;
> > > >     * Set a xen_guest_handle in a type safe manner, ensuring that the
> > > >     * data pointer has been correctly allocated.
> > > >     */
> > > > -#undef set_xen_guest_handle
> > > > -#define set_xen_guest_handle(_hnd, _val)                        \
> > > > +#undef set_xen_guest_handle_offset
> > > This isn't strictly speaking needed since you aren't trying to override
> > > a version of this interface provided by the hypervisor headers.
> > >
> > > > +#define set_xen_guest_handle_offset(_hnd, _val, _off)           \
> > > >        do {                                                        \
> > > >            xc_hypercall_buffer_t _hcbuf_hnd1;                      \
> > > >            typeof(XC__HYPERCALL_BUFFER_NAME(_val)) *_hcbuf_hnd2 =  \
> > > >                    HYPERCALL_BUFFER(_val);                         \
> > > >            (void) (&_hcbuf_hnd1 == _hcbuf_hnd2);                   \
> > > > -        set_xen_guest_handle_raw(_hnd, (_hcbuf_hnd2)->hbuf);    \
> > > > +        set_xen_guest_handle_raw(_hnd,                          \
> > > > +                                 (_hcbuf_hnd2)->hbuf + (_off)); \
> > > The hypervisor side guest_handle_add_offset equivalents operate in
> > > multiples of the type. If we can arrange that here too then I think that
> > > would be good.
> > >
> > > I think that means
> > > +                                 (_hcbuf_hnd2)->hbuf + 
> > > ((sizeof(*_val)*(_off))); \
> > >
> > > Then the caller becomes e.g.
> > >
> > > set_xen_guest_handle_offset(sysctl.u.pcitopoinfo.devs, devs, processed);
> > >
> > > which is much nicer IMHO.
> > That's what I started with.
> >
> > This, however, breaks at least two things:
> > --  Callers that pass HYPERCALL_BUFFER_NULL to set_xen_guest_handle().
> > This I think can be fixed by declaring a void  *HYPERCALL_BUFFER_NULL
> > (currenty HYPERCALL_BUFFER_NULL is just a syntactic building block)
> >
> > -- DECLARE_NAMED_HYPERCALL_BOUNCE. This makes _val a non-existing symbol
> > (it creates a xc__hypercall_buffer_val). That I am not sure I know how
> > to fix. Add type size filed to struct xc_hypercall_buffer?
> How about refactoring such that this macro becomes an internal helper
> used by both set_xen_guest_handle and set_xen_guest_handle_offset and
> takes the (_hcbuf_hnd2)->hbuf or (_hcbuf_hnd2)->hbuf +
> ((sizeof(*_val)*(_off)) as appropriate? (iow don't implement set as
> set_offset(0))
But that will preclude us from using _offset() variant of the macro for 
variables declared with DECLARE_NAMED_HYPERCALL_BOUNCE (and possibly 
others, I haven't checked), won't it? We don't have any such uses now 
but nevertheless...
-boris

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