Re: [Xen-devel] RFC: 32 bits as smallest atomic size.

[Jimi Xenidis <jimix@xxxxxxxxxxxxxx>]

>>>>> "KF" == Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> writes:

 KF> I expect we can fix that up in the ppc macros: if the atomic access is 
 KF> sub-32-bit aligned then round the address down to 32-bit boundary and 
 KF> do a 32-bit cmpxchg.

A few issues:
  1) this assumes that the quantity is wholy contained in our atomic
     unit, by using packed there is no way to guarantee that.

  2) it would have to be a runtime decision on all PPC atomic
     operations that could fail with no real failure path available.

We could also program it out, consider this alternative:
    typedef struct {
        struct {
            u16     flags;      /* 0 */
            domid_t domid;      /* 2 */
        } atomic;
        u32     frame;      /* 4 */
    } grant_entry_t; /* 8 bytes */

Few things to note:
  1) packed is necessary since the ABI will do the right thing
  2) would simplify the code at grant_table.c:162
  3) will have better performance since all access are guranteed to be
     aligned.

I understand the usefulness of packed, but its gratuitous use
unecessarily complicates other architectures, and can hurt even x86 if
it results in a data that "straddles" cachelines.

-JX


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