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

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

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...


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