On Tue, 5 Jan 2016 15:04:43 +0200 "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > On Tue, Jan 05, 2016 at 01:08:52PM +0100, Martin Schwidefsky wrote: > > On Tue, 5 Jan 2016 11:30:19 +0200 > > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > > > > On Tue, Jan 05, 2016 at 09:13:19AM +0100, Martin Schwidefsky wrote: > > > > On Mon, 4 Jan 2016 22:18:58 +0200 > > > > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > > > > > > > > On Mon, Jan 04, 2016 at 02:45:25PM +0100, Peter Zijlstra wrote: > > > > > > On Thu, Dec 31, 2015 at 09:08:38PM +0200, Michael S. Tsirkin wrote: > > > > > > > This defines __smp_xxx barriers for s390, > > > > > > > for use by virtualization. > > > > > > > > > > > > > > Some smp_xxx barriers are removed as they are > > > > > > > defined correctly by asm-generic/barriers.h > > > > > > > > > > > > > > Note: smp_mb, smp_rmb and smp_wmb are defined as full barriers > > > > > > > unconditionally on this architecture. > > > > > > > > > > > > > > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx> > > > > > > > Acked-by: Arnd Bergmann <arnd@xxxxxxxx> > > > > > > > --- > > > > > > > arch/s390/include/asm/barrier.h | 15 +++++++++------ > > > > > > > 1 file changed, 9 insertions(+), 6 deletions(-) > > > > > > > > > > > > > > diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h > > > > > > > index c358c31..fbd25b2 100644 > > > > > > > --- a/arch/s390/include/asm/barrier.h > > > > > > > +++ b/arch/s390/include/asm/barrier.h > > > > > > > @@ -26,18 +26,21 @@ > > > > > > > #define wmb() barrier() > > > > > > > #define dma_rmb() mb() > > > > > > > #define dma_wmb() mb() > > > > > > > -#define smp_mb() mb() > > > > > > > -#define smp_rmb() rmb() > > > > > > > -#define smp_wmb() wmb() > > > > > > > - > > > > > > > -#define smp_store_release(p, v) \ > > > > > > > +#define __smp_mb() mb() > > > > > > > +#define __smp_rmb() rmb() > > > > > > > +#define __smp_wmb() wmb() > > > > > > > +#define smp_mb() __smp_mb() > > > > > > > +#define smp_rmb() __smp_rmb() > > > > > > > +#define smp_wmb() __smp_wmb() > > > > > > > > > > > > Why define the smp_*mb() primitives here? Would not the inclusion of > > > > > > asm-generic/barrier.h do this? > > > > > > > > > > No because the generic one is a nop on !SMP, this one isn't. > > > > > > > > > > Pls note this patch is just reordering code without making > > > > > functional changes. > > > > > And at the moment, on s390 smp_xxx barriers are always non empty. > > > > > > > > The s390 kernel is SMP to 99.99%, we just didn't bother with a > > > > non-smp variant for the memory-barriers. If the generic header > > > > is used we'd get the non-smp version for free. It will save a > > > > small amount of text space for CONFIG_SMP=n. > > > > > > OK, so I'll queue a patch to do this then? > > > > Yes please. > > OK, I'll add a patch on top in v3. Good, with this addition: Acked-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx> > > > Just to make sure: the question would be, are smp_xxx barriers ever used > > > in s390 arch specific code to flush in/out memory accesses for > > > synchronization with the hypervisor? > > > > > > I went over s390 arch code and it seems to me the answer is no > > > (except of course for virtio). > > > > Correct. Guest to host communication either uses instructions which > > imply a memory barrier or QDIO which uses atomics. > > And atomics imply a barrier on s390, right? Yes they do. > > > But I also see a lot of weirdness on this architecture. > > > > Mostly historical, s390 actually is one of the easiest architectures in > > regard to memory barriers. > > > > > I found these calls: > > > > > > arch/s390/include/asm/bitops.h: smp_mb__before_atomic(); > > > arch/s390/include/asm/bitops.h: smp_mb(); > > > > > > Not used in arch specific code so this is likely OK. > > > > This has been introduced with git commit 5402ea6af11dc5a9, the smp_mb > > and smp_mb__before_atomic are used in clear_bit_unlock and > > __clear_bit_unlock which are 1:1 copies from the code in > > include/asm-generic/bitops/lock.h. Only test_and_set_bit_lock differs > > from the generic implementation. > > something to keep in mind, but > I'd rather not touch bitops at the moment - this patchset is already too big. With the conversion smp_mb__before_atomic to a barrier() it does the correct thing. I don't think that any chance is necessary. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.
