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

[Michael S. Tsirkin]

On Tue, Jan 05, 2016 at 04:39:37PM +0100, Christian Borntraeger wrote:
> On 01/05/2016 10:30 AM, Michael S. Tsirkin wrote:
> 
> > 
> > arch/s390/kernel/vdso.c:smp_mb();
> > 
> > Looking at
> > Author: Christian Borntraeger 
> > Date:   Fri Sep 11 16:23:06 2015 +0200
> > 
> > s390/vdso: use correct memory barrier
> > 
> > By definition smp_wmb only orders writes against writes. (Finish all
> > previous writes, and do not start any future write). To protect the
> > vdso init code against early reads on other CPUs, let's use a full
> > smp_mb at the end of vdso init. As right now smp_wmb is implemented
> > as full serialization, this needs no stable backport, but this 
> > change
> > will be necessary if we reimplement smp_wmb.
> > 
> > ok from hypervisor point of view, but it's also strange:
> > 1. why isn't this paired with another mb somewhere?
> >this seems to violate barrier pairing rules.
> > 2. how does smp_mb protect against early reads on other CPUs?
> >It normally does not: it orders reads from this CPU versus writes
> >from same CPU. But init code does not appear to read anything.
> >Maybe this is some s390 specific trick?
> > 
> > I could not figure out the above commit.
> 
> It was probably me misreading the code. I change a wmb into a full mb here
> since I was changing the defintion of wmb to a compiler barrier. I tried to
> fixup all users of wmb that really pair with other code. I assumed that there
> must be some reader (as there was a wmb before) but I could not figure out
> which. So I just played safe here.
> 
> But it probably can be removed.
> 
> > arch/s390/kvm/kvm-s390.c:   smp_mb();
> 
> This can go. If you have a patch, I can carry that via the kvms390 tree,
> or I will spin a new patch with you as suggested-by.
> 
> Christian

I have both, will post shortly.

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

[Martin Schwidefsky]

On Tue, 5 Jan 2016 11:30:19 +0200
"Michael S. Tsirkin"  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"  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 
> > > > > Acked-by: Arnd Bergmann 
> > > > > ---
> > > > >  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.
 
> 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.

> 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.

> arch/s390/kernel/vdso.c:smp_mb();
> 
> Looking at
>   Author: Christian Borntraeger 
>   Date:   Fri Sep 11 16:23:06 2015 +0200
> 
>   s390/vdso: use correct memory barrier
> 
>   By definition smp_wmb only orders writes against writes. (Finish all
>   previous writes, and do not start any future write). To protect the
>   vdso init code against early reads on other CPUs, let's use a full
>   smp_mb at the end of vdso init. As right now smp_wmb is implemented
>   as full serialization, this needs no stable backport, but this 
> change
>   will be necessary if we reimplement smp_wmb.
> 
> ok from hypervisor point of view, but it's also strange:
> 1. why isn't this paired with another mb somewhere?
>this seems to violate barrier pairing rules.
> 2. how does smp_mb protect against early reads on other CPUs?
>It normally does not: it orders reads from this CPU versus writes
>from same CPU. But init code does not appear to read anything.
>Maybe this is some s390 specific trick?
> 
> I could not figure out the above commit.

That smp_mb can be removed. The initial s390 vdso code is heavily 

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

[Michael S. Tsirkin]

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 
> > Acked-by: Arnd Bergmann 
> > ---
> >  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.

Some of this could be sub-optimal, but
since on s390 Linux always runs on a hypervisor,
I am not sure it's safe to use the generic version -
in other words, it just might be that for s390 smp_ and virt_
barriers must be equivalent.

If in fact this turns out to be wrong, I can pick up
a patch to change this, but I'd rather make this
a patch on top so that my patches are testable
just by compiling and comparing the binary.

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