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

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

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))




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