x86-64, rwsem: Avoid store forwarding hazard in __downgrade_write

The Intel Architecture Optimization Reference Manual states that a short
load that follows a long store to the same object will suffer a store
forwading penalty, particularly if the two accesses use different addresses.
Trivially, a long load that follows a short store will also suffer a penalty.

__downgrade_write() in rwsem incurs both penalties:  the increment operation
will not be able to reuse a recently-loaded rwsem value, and its result will
not be reused by any recently-following rwsem operation.

A comment in the code states that this is because 64-bit immediates are
special and expensive; but while they are slightly special (only a single
instruction allows them), they aren't expensive: a test shows that two loops,
one loading a 32-bit immediate and one loading a 64-bit immediate, both take
1.5 cycles per iteration.

Fix this by changing __downgrade_write to use the same add instruction on
i386 and on x86_64, so that it uses the same operand size as all the other
rwsem functions.

Signed-off-by: Avi Kivity <[email protected]>
LKML-Reference: <[email protected]>
Signed-off-by: H. Peter Anvin <[email protected]>

0 files changed, 0 insertions, 0 deletions