Offset 128 overlaps the last word of the redzone.
Use 132 which is always beyond that.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
MFENCE appears to be way slower than a locked instruction - let's use
LOCK ADD unconditionally, as we always did on old 32-bit.
Performance testing results:
perf stat -r 10 -- ./virtio_ring_0_9 --sleep --host-affinity 0 --guest-affinity 0
Before:
0.922565990 seconds time elapsed ( +- 1.15% )
After:
0.578667024 seconds time elapsed ( +- 1.21% )
i.e. about ~60% faster.
Just poking at SP would be the most natural, but if we then read the
value from SP, we get a false dependency which will slow us down.
This was noted in this article:
http://shipilev.net/blog/2014/on-the-fence-with-dependencies/
And is easy to reproduce by sticking a barrier in a small non-inline
function.
So let's use a negative offset - which avoids this problem since we
build with the red zone disabled.
For userspace, use an address just below the redzone.
The one difference between LOCK ADD and MFENCE is that LOCK ADD does
not affect CLFLUSH, previous patches converted all uses of CLFLUSH to
call mb(), such that changes to smp_mb() won't affect it.
Update mb/rmb/wmb() on 32-bit to use the negative offset, too, for
consistency.
As a follow-up, it might be worth considering switching users
of CLFLUSH to another API (e.g. clflush_mb()?) - we will
then be able to convert mb() to smp_mb() again.
Also arguably, GCC should switch to use LOCK ADD for __sync_synchronize().
This might be worth pursuing separately.
Suggested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: qemu-devel@nongnu.org
Cc: virtualization@lists.linux-foundation.org
Link: http://lkml.kernel.org/r/1509118355-4890-1-git-send-email-mst@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Make ringtest work on s390 too.
Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Acked-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Provide new primitives used_empty/avail_empty and
build poll_avail/poll_used on top of it.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
This adds micro-benchmarks useful for tuning virtio ring layouts.
Three layouts are currently implemented:
- virtio 0.9 compatible one
- an experimental extension bypassing the ring index, polling ring
itself instead
- an experimental extension bypassing avail and used ring completely
Typical use:
sh run-on-all.sh perf stat -r 10 --log-fd 1 -- ./ring
It doesn't depend on the kernel directly, but it's handy
to have as much virtio stuff as possible in one tree.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Michael S. Tsirkin