Re: [Xen-devel] [PATCH v2 22/32] s390: define __smp_xxx

[Martin Schwidefsky <schwidefsky@xxxxxxxxxx>]

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.


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