Jimi Xenidis
2006-Aug-11 16:36 UTC
[Xen-devel] [PATCH][XEN] Use a union to pack the dual-short combos in an endian neutral way.
The first to members of a grant entry are updated as a combined pair.
The following union patch uses a union so updated can happen in an
endian neutral fashion.
Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>
diff -r 1f611b58729f xen/common/grant_table.c
--- a/xen/common/grant_table.c Wed Aug 09 18:53:00 2006 -0400
+++ b/xen/common/grant_table.c Fri Aug 11 12:29:48 2006 -0400
@@ -32,6 +32,17 @@
#include <xen/guest_access.h>
#include <acm/acm_hooks.h>
+/* The first to members of a grant entry are updated as a combined
+ * pair. The following union allows that to happen in an endian
+ * neutral fashion. */
+union grant_combo {
+ uint32_t word;
+ struct {
+ uint16_t flags;
+ domid_t domid;
+ } shorts;
+};
+
#define PIN_FAIL(_lbl, _rc, _f, _a...) \
do { \
DPRINTK( _f, ## _a ); \
@@ -177,7 +188,7 @@ __gnttab_map_grant_ref(
*/
for ( ; ; )
{
- u32 scombo, prev_scombo, new_scombo;
+ union grant_combo scombo, prev_scombo, new_scombo;
if ( unlikely((sflags & GTF_type_mask) != GTF_permit_access) ||
unlikely(sdom != led->domain->domain_id) )
@@ -186,22 +197,25 @@ __gnttab_map_grant_ref(
sflags, sdom, led->domain->domain_id);
/* Merge two 16-bit values into a 32-bit combined update. */
- /* NB. Endianness! */
- scombo = ((u32)sdom << 16) | (u32)sflags;
-
- new_scombo = scombo | GTF_reading;
+ scombo.shorts.flags = sflags;
+ scombo.shorts.domid = sdom;
+
+ new_scombo = scombo;
+ new_scombo.shorts.flags |= GTF_reading;
+
if ( !(op->flags & GNTMAP_readonly) )
{
- new_scombo |= GTF_writing;
+ new_scombo.shorts.flags |= GTF_writing;
if ( unlikely(sflags & GTF_readonly) )
PIN_FAIL(unlock_out, GNTST_general_error,
"Attempt to write-pin a r/o grant
entry.\n");
}
- prev_scombo = cmpxchg((u32 *)&sha->flags, scombo,
new_scombo);
+ prev_scombo.word = cmpxchg((u32 *)&sha->flags,
+ scombo.word, new_scombo.word);
/* Did the combined update work (did we see what we expected?). */
- if ( likely(prev_scombo == scombo) )
+ if ( likely(prev_scombo.word == scombo.word) )
break;
if ( retries++ == 4 )
@@ -209,9 +223,8 @@ __gnttab_map_grant_ref(
"Shared grant entry is unstable.\n");
/* Didn''t see what we expected. Split out the seen flags
& dom. */
- /* NB. Endianness! */
- sflags = (u16)prev_scombo;
- sdom = (u16)(prev_scombo >> 16);
+ sflags = prev_scombo.shorts.flags;
+ sdom = prev_scombo.shorts.domid;
}
if ( !act->pin )
@@ -532,7 +545,7 @@ gnttab_prepare_for_transfer(
struct grant_entry *sha;
domid_t sdom;
u16 sflags;
- u32 scombo, prev_scombo;
+ union grant_combo scombo, prev_scombo, tmp_scombo;
int retries = 0;
if ( unlikely((rgt = rd->grant_table) == NULL) ||
@@ -561,14 +574,16 @@ gnttab_prepare_for_transfer(
}
/* Merge two 16-bit values into a 32-bit combined update. */
- /* NB. Endianness! */
- scombo = ((u32)sdom << 16) | (u32)sflags;
-
- prev_scombo = cmpxchg((u32 *)&sha->flags, scombo,
- scombo | GTF_transfer_committed);
+ scombo.shorts.flags = sflags;
+ scombo.shorts.domid = sdom;
+
+ tmp_scombo = scombo;
+ tmp_scombo.shorts.flags |= GTF_transfer_committed;
+ prev_scombo.word = cmpxchg((u32 *)&sha->flags,
+ scombo.word, tmp_scombo.word);
/* Did the combined update work (did we see what we expected?). */
- if ( likely(prev_scombo == scombo) )
+ if ( likely(prev_scombo.word == scombo.word) )
break;
if ( retries++ == 4 )
@@ -578,9 +593,8 @@ gnttab_prepare_for_transfer(
}
/* Didn''t see what we expected. Split out the seen flags &
dom. */
- /* NB. Endianness! */
- sflags = (u16)prev_scombo;
- sdom = (u16)(prev_scombo >> 16);
+ sflags = prev_scombo.shorts.flags;
+ sdom = prev_scombo.shorts.domid;
}
spin_unlock(&rgt->lock);
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Keir Fraser
2006-Aug-13 17:46 UTC
Re: [Xen-devel] [PATCH][XEN] Use a union to pack the dual-short combos in an endian neutral way.
I think it''s an improvement in readability. Fine by me. -- Keir On 13/8/06 6:50 pm, "Jimi Xenidis" <jimix@watson.ibm.com> wrote:> Any comments on how this was done? > Shall I continue to apply this method to the new grant-table copy > operation? > -JX > On Aug 11, 2006, at 12:36 PM, Jimi Xenidis wrote: > >> >> The first to members of a grant entry are updated as a combined pair. >> The following union patch uses a union so updated can happen in an >> endian neutral fashion. >> >> Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>_______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Jimi Xenidis
2006-Aug-13 17:50 UTC
Re: [Xen-devel] [PATCH][XEN] Use a union to pack the dual-short combos in an endian neutral way.
Any comments on how this was done? Shall I continue to apply this method to the new grant-table copy operation? -JX On Aug 11, 2006, at 12:36 PM, Jimi Xenidis wrote:> > The first to members of a grant entry are updated as a combined pair. > The following union patch uses a union so updated can happen in an > endian neutral fashion. > > Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com> > > > diff -r 1f611b58729f xen/common/grant_table.c > --- a/xen/common/grant_table.c Wed Aug 09 18:53:00 2006 -0400 > +++ b/xen/common/grant_table.c Fri Aug 11 12:29:48 2006 -0400 > @@ -32,6 +32,17 @@ > #include <xen/guest_access.h> > #include <acm/acm_hooks.h> > > +/* The first to members of a grant entry are updated as a combined > + * pair. The following union allows that to happen in an endian > + * neutral fashion. */ > +union grant_combo { > + uint32_t word; > + struct { > + uint16_t flags; > + domid_t domid; > + } shorts; > +}; > + > #define PIN_FAIL(_lbl, _rc, _f, _a...) \ > do { \ > DPRINTK( _f, ## _a ); \ > @@ -177,7 +188,7 @@ __gnttab_map_grant_ref( > */ > for ( ; ; ) > { > - u32 scombo, prev_scombo, new_scombo; > + union grant_combo scombo, prev_scombo, new_scombo; > > if ( unlikely((sflags & GTF_type_mask) != > GTF_permit_access) || > unlikely(sdom != led->domain->domain_id) ) > @@ -186,22 +197,25 @@ __gnttab_map_grant_ref( > sflags, sdom, led->domain->domain_id); > > /* Merge two 16-bit values into a 32-bit combined > update. */ > - /* NB. Endianness! */ > - scombo = ((u32)sdom << 16) | (u32)sflags; > - > - new_scombo = scombo | GTF_reading; > + scombo.shorts.flags = sflags; > + scombo.shorts.domid = sdom; > + > + new_scombo = scombo; > + new_scombo.shorts.flags |= GTF_reading; > + > if ( !(op->flags & GNTMAP_readonly) ) > { > - new_scombo |= GTF_writing; > + new_scombo.shorts.flags |= GTF_writing; > if ( unlikely(sflags & GTF_readonly) ) > PIN_FAIL(unlock_out, GNTST_general_error, > "Attempt to write-pin a r/o grant > entry.\n"); > } > > - prev_scombo = cmpxchg((u32 *)&sha->flags, scombo, > new_scombo); > + prev_scombo.word = cmpxchg((u32 *)&sha->flags, > + scombo.word, new_scombo.word); > > /* Did the combined update work (did we see what we > expected?). */ > - if ( likely(prev_scombo == scombo) ) > + if ( likely(prev_scombo.word == scombo.word) ) > break; > > if ( retries++ == 4 ) > @@ -209,9 +223,8 @@ __gnttab_map_grant_ref( > "Shared grant entry is unstable.\n"); > > /* Didn''t see what we expected. Split out the seen > flags & dom. */ > - /* NB. Endianness! */ > - sflags = (u16)prev_scombo; > - sdom = (u16)(prev_scombo >> 16); > + sflags = prev_scombo.shorts.flags; > + sdom = prev_scombo.shorts.domid; > } > > if ( !act->pin ) > @@ -532,7 +545,7 @@ gnttab_prepare_for_transfer( > struct grant_entry *sha; > domid_t sdom; > u16 sflags; > - u32 scombo, prev_scombo; > + union grant_combo scombo, prev_scombo, tmp_scombo; > int retries = 0; > > if ( unlikely((rgt = rd->grant_table) == NULL) || > @@ -561,14 +574,16 @@ gnttab_prepare_for_transfer( > } > > /* Merge two 16-bit values into a 32-bit combined update. */ > - /* NB. Endianness! */ > - scombo = ((u32)sdom << 16) | (u32)sflags; > - > - prev_scombo = cmpxchg((u32 *)&sha->flags, scombo, > - scombo | GTF_transfer_committed); > + scombo.shorts.flags = sflags; > + scombo.shorts.domid = sdom; > + > + tmp_scombo = scombo; > + tmp_scombo.shorts.flags |= GTF_transfer_committed; > + prev_scombo.word = cmpxchg((u32 *)&sha->flags, > + scombo.word, tmp_scombo.word); > > /* Did the combined update work (did we see what we > expected?). */ > - if ( likely(prev_scombo == scombo) ) > + if ( likely(prev_scombo.word == scombo.word) ) > break; > > if ( retries++ == 4 ) > @@ -578,9 +593,8 @@ gnttab_prepare_for_transfer( > } > > /* Didn''t see what we expected. Split out the seen flags & > dom. */ > - /* NB. Endianness! */ > - sflags = (u16)prev_scombo; > - sdom = (u16)(prev_scombo >> 16); > + sflags = prev_scombo.shorts.flags; > + sdom = prev_scombo.shorts.domid; > } > > spin_unlock(&rgt->lock); > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xensource.com > http://lists.xensource.com/xen-devel_______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel