Re: [Xen-devel] [PATCH v3 00/41] arch: barrier cleanup + barriers for virt

[Peter Zijlstra]

On Sun, Jan 10, 2016 at 04:16:22PM +0200, Michael S. Tsirkin wrote:
> I parked this in vhost tree for now, though the inclusion of patch 1 from tip
> creates a merge conflict - but one that is trivial to resolve.
> 
> So I intend to just merge it all through my tree, including the
> duplicate patch, and assume conflict will be resolved.
> 
> I would really appreciate some feedback on arch bits (especially the x86 
> bits),
> and acks for merging this through the vhost tree.

Thanks for doing this, looks good to me.

Acked-by: Peter Zijlstra (Intel) 

___
Xen-devel mailing list
Xen-devel@lists.xen.org
http://lists.xen.org/xen-devel

[Xen-devel] [PATCH v3 00/41] arch: barrier cleanup + barriers for virt

[Michael S. Tsirkin]

Changes since v2:
- extended checkpatch tests for barriers, and added patches
teaching it to warn about incorrect usage of barriers
(__smp_xxx barriers are for use by asm-generic code only),
should help prevent misuse by arch code
to address comments by Russell King
- patched more instances of xen to use virt_ barriers
as suggested by  Stefano Stabellini
- implemented a 2 byte xchg on sh instead of hacking around it
as suggested by Peter Zijlstra and  Rich Felker
- added a patch to drop some s390 arch-specific smp_xxx barriers - 
generic
versions are more efficient
as suggested by Peter Zijlstra and Martin Schwidefsky
- added a patch to replace before/after atomic barriers with barrier()
on s390 as suggested by Peter Zijlstra and Martin Schwidefsky
- included acks from multiple arch maintainers
thanks a lot for the review!

Changes since v1:
- replaced an asm-generic patch with an equivalent patch already in tip
- add wrappers with virt_ prefix for better code annotation,
  as suggested by David Miller
- dropped XXX in patch names as this makes vger choke, Cc all relevant
  mailing lists on all patches (not personal email, as the list becomes
  too long then)

I parked this in vhost tree for now, though the inclusion of patch 1 from tip
creates a merge conflict - but one that is trivial to resolve.

So I intend to just merge it all through my tree, including the
duplicate patch, and assume conflict will be resolved.

I would really appreciate some feedback on arch bits (especially the x86 bits),
and acks for merging this through the vhost tree.

Thanks!

What really started me off is trying to cleanup some virt code, as suggested by
Peter, who said
> You could of course go fix that instead of mutilating things into
> sort-of functional state.

This work is needed for virtio, so it's probably easiest to
merge it through my tree - is this fine by everyone?

Note to arch maintainers: please don't cherry-pick patches out of this patchset
as it's been structured in this order to avoid breaking bisect.
Please send acks instead!

=

Sometimes, virtualization is weird. For example, virtio does this 
(conceptually):

#ifdef CONFIG_SMP
smp_mb();
#else
mb();
#endif

Similarly, Xen calls mb() when it's not doing any MMIO at all.

Of course it's wrong in the sense that it's suboptimal. What we would really
like is to have, on UP, exactly the same barrier as on SMP.  This is because a
UP guest can run on an SMP host.

But Linux doesn't provide this ability: if CONFIG_SMP is not defined is
optimizes most barriers out to a compiler barrier.

Consider for example x86: what we want is xchg (NOT mfence - there's no real IO
going on here - just switching out of the VM - more like a function call
really) but if built without CONFIG_SMP smp_store_mb does not include this.

Virt in general is probably the only use-case, because this really is an
artifact of interfacing with an SMP host while running an UP kernel,
but since we have (at least) two users, it seems to make sense to
put these APIs in a central place.

In fact, smp_ barriers are stubs on !SMP, so they can be defined as follows:

arch/XXX/include/asm/barrier.h:

#define __smp_mb() DOSOMETHING

include/asm-generic/barrier.h:

#ifdef CONFIG_SMP
#define smp_mb() __smp_mb()
#else
#define smp_mb() barrier()
#endif

This has the benefit of cleaning out a bunch of duplicated
ifdefs on a bunch of architectures - this patchset brings
about a net reduction in LOC, more than compensated for
later by performance enhancements, extra documentation and tools :)

Then virt can use __smp_XXX when talking to an SMP host.
To make those users explicit, this patchset adds virt_xxx wrappers
for them.

Touching all archs is a tad tedious, but its fairly straight forward.

The patchset is structured as follows:


-. Patch 1 fixes a bug in asm-generic.
   It is already in tip, included here for completeness.

-. Patches 2-12 make sure barrier.h on all remaining
   architectures includes asm-generic/barrier.h:
   after the change in Patch 1, code there matches
   asm-generic/barrier.h almost verbatim.
   Minor code tweaks were required in a couple of places.
   Macros duplicated from asm-generic/barrier.h are dropped
   in the process.

After all that preparatory work, we are getting to the actual change.

-. Patch 13 adds generic smp_XXX wrappers in asm-generic:
   these select __smp_XXX or barrier() depending on CONFIG_SMP

-. Patches 14-27 change all architectures to
   define __smp_XXX macros; the generic code in asm-generic/barrier.h
   then defines smp_XXX macros

   I compiled the affected arches before and after the changes,
   dumped the .text section (using objdump -O binary) and
   made sure that the object code is exactly identical
   before and after the change.

   Note: