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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 25 Mar 2005, at 23:49, Jimi Xenidis wrote:

> > > > > > "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.
We always naturally align fields, even in packed structs. The compiler 
doesn't do it for us, but we check it manually.
>   2) it would have to be a runtime decision on all PPC atomic
>      operations that could fail with no real failure path available.
It may have to be a run-time decision unless we can come up with some 
clever trick. I don't understand what you mean about there being no 
failure path.
> 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 */
This may make sense since we do update those two fields as a pair. We 
have a big update to the grant table code about to be checked in. We 
shoudl hold off doing this kind of fix until after the update has been 
committed.

> 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.
We always naturally align fields. We may do *subword* accesses 
sometimes, but they are always naturally aligned and will never 
straddle a cacheline. I think that subword accesses should get fixed up 
in your ppc macros. In teh specific case of the grant table structures 
let's look closer once teh big update is in the tree.
 -- Keir



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