Re: [PATCH v2 17/32] arm: define __smp_xxx

[Michael S. Tsirkin]

On Mon, Jan 04, 2016 at 02:36:58PM +0100, Peter Zijlstra wrote:
> On Sun, Jan 03, 2016 at 11:12:44AM +0200, Michael S. Tsirkin wrote:
> > On Sat, Jan 02, 2016 at 11:24:38AM +, Russell King - ARM Linux wrote:
> 
> > > My only concern is that it gives people an additional handle onto a
> > > "new" set of barriers - just because they're prefixed with __*
> > > unfortunately doesn't stop anyone from using it (been there with
> > > other arch stuff before.)
> > > 
> > > I wonder whether we should consider making the smp memory barriers
> > > inline functions, so these __smp_xxx() variants can be undef'd
> > > afterwards, thereby preventing drivers getting their hands on these
> > > new macros?
> > 
> > That'd be tricky to do cleanly since asm-generic depends on
> > ifndef to add generic variants where needed.
> > 
> > But it would be possible to add a checkpatch test for this.
> 
> Wasn't the whole purpose of these things for 'drivers' (namely
> virtio/xen hypervisor interaction) to use these?

My take out from discussion with you was that virtualization is probably
the only valid use-case.  So at David Miller's suggestion there's a
patch later in the series that adds virt_ wrappers and these are
then used by virtio xen and later maybe others.

> And I suppose most of virtio would actually be modules, so you cannot do
> what I did with preempt_enable_no_resched() either.
> 
> But yes, it would be good to limit the use of these things.

Right so the trick is checkpatch warns about use of
__smp_xxx and hopefully people are not crazy enough
to use virt_xxx variants for non-virtual drivers.

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Re: [PATCH v2 17/32] arm: define __smp_xxx

[Peter Zijlstra]

On Sun, Jan 03, 2016 at 11:12:44AM +0200, Michael S. Tsirkin wrote:
> On Sat, Jan 02, 2016 at 11:24:38AM +, Russell King - ARM Linux wrote:

> > My only concern is that it gives people an additional handle onto a
> > "new" set of barriers - just because they're prefixed with __*
> > unfortunately doesn't stop anyone from using it (been there with
> > other arch stuff before.)
> > 
> > I wonder whether we should consider making the smp memory barriers
> > inline functions, so these __smp_xxx() variants can be undef'd
> > afterwards, thereby preventing drivers getting their hands on these
> > new macros?
> 
> That'd be tricky to do cleanly since asm-generic depends on
> ifndef to add generic variants where needed.
> 
> But it would be possible to add a checkpatch test for this.

Wasn't the whole purpose of these things for 'drivers' (namely
virtio/xen hypervisor interaction) to use these?

And I suppose most of virtio would actually be modules, so you cannot do
what I did with preempt_enable_no_resched() either.

But yes, it would be good to limit the use of these things.
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Re: [PATCH v2 17/32] arm: define __smp_xxx

[Russell King - ARM Linux]

On Thu, Dec 31, 2015 at 09:07:59PM +0200, Michael S. Tsirkin wrote:
> This defines __smp_xxx barriers for arm,
> for use by virtualization.
> 
> smp_xxx barriers are removed as they are
> defined correctly by asm-generic/barriers.h
> 
> This reduces the amount of arch-specific boiler-plate code.
> 
> Signed-off-by: Michael S. Tsirkin 
> Acked-by: Arnd Bergmann 

In combination with patch 14, this looks like it should result in no
change to the resulting code.

Acked-by: Russell King 

My only concern is that it gives people an additional handle onto a
"new" set of barriers - just because they're prefixed with __*
unfortunately doesn't stop anyone from using it (been there with
other arch stuff before.)

I wonder whether we should consider making the smp memory barriers
inline functions, so these __smp_xxx() variants can be undef'd
afterwards, thereby preventing drivers getting their hands on these
new macros?

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