Jimi Xenidis
2006-Aug-14 14:03 UTC
[Xen-devel] [PATCH][XEN] Take 2: Use a union to pack the dual-short combos in an endian neutral way.
The first two members of a grant entry are updated as a combined pair.
The following patch uses a union so updates can happen in an
endian neutral fashion.
Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>
---
diff -r f328519053f5 xen/common/grant_table.c
--- a/xen/common/grant_table.c Mon Aug 14 10:58:02 2006 +0100
+++ b/xen/common/grant_table.c Mon Aug 14 09:59:48 2006 -0400
@@ -33,6 +33,17 @@
#include <xen/domain_page.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 ); \
@@ -178,7 +189,7 @@ __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) )
@@ -187,22 +198,25 @@ __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 )
@@ -210,9 +224,8 @@ __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 )
@@ -533,7 +546,7 @@ 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) ||
@@ -562,14 +575,16 @@ 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 )
@@ -579,9 +594,8 @@ 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);
@@ -764,33 +778,38 @@ __acquire_grant_for_copy(
for ( ; ; )
{
- u32 scombo;
- u32 prev_scombo;
- u32 new_scombo;
+ union grant_combo scombo, prev_scombo, new_scombo;
if ( unlikely((sflags & GTF_type_mask) != GTF_permit_access ||
sdom != current->domain->domain_id ) )
PIN_FAIL(unlock_out, GNTST_general_error,
"Bad flags (%x) or dom (%d). (NB. expected dom
%d)\n",
sflags, sdom, current->domain->domain_id);
- scombo = ((u32)sdom << 16) | (u32)sflags;
- new_scombo = scombo | GTF_reading;
+
+ /* Merge two 16-bit values into a 32-bit combined update. */
+ scombo.shorts.flags = sflags;
+ scombo.shorts.domid = sdom;
+
+ new_scombo = scombo;
+ new_scombo.shorts.flags |= GTF_reading;
+
if ( !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);
- if ( likely(prev_scombo == scombo) )
+ prev_scombo.word = cmpxchg((u32 *)&sha->flags,
+ scombo.word, new_scombo.word);
+ if ( likely(prev_scombo.word == scombo.word) )
break;
if ( retries++ == 4 )
PIN_FAIL(unlock_out, GNTST_general_error,
"Shared grant entry is unstable.\n");
- sflags = (u16)prev_scombo;
- sdom = (u16)(prev_scombo >> 16);
+ sflags = prev_scombo.shorts.flags;
+ sdom = prev_scombo.shorts.flags;
}
if ( !act->pin )
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