Re: [PATCH RFC 3/4] barriers: convert a control to a data dependency

[Michael S. Tsirkin]

On Mon, Jan 07, 2019 at 11:02:36AM -0800, Paul E. McKenney wrote:
> On Mon, Jan 07, 2019 at 08:36:36AM -0500, Michael S. Tsirkin wrote:
> > On Mon, Jan 07, 2019 at 10:46:10AM +0100, Peter Zijlstra wrote:
> > > On Sun, Jan 06, 2019 at 11:23:07PM -0500, Michael S. Tsirkin wrote:
> > > > On Mon, Jan 07, 2019 at 11:58:23AM +0800, Jason Wang wrote:
> > > > > On 2019/1/3 äå4:57, Michael S. Tsirkin wrote:
> > > 
> > > > > > +#if defined(COMPILER_HAS_OPTIMIZER_HIDE_VAR) && \
> > > > > > +	!defined(ARCH_NEEDS_READ_BARRIER_DEPENDS)
> > > > > > +
> > > > > > +#define dependent_ptr_mb(ptr, val) ({					\
> > > > > > +	long dependent_ptr_mb_val = (long)(val);			\
> > > > > > +	long dependent_ptr_mb_ptr = (long)(ptr) - dependent_ptr_mb_val;	\
> > > > > > +									\
> > > > > > +	BUILD_BUG_ON(sizeof(val) > sizeof(long));			\
> > > > > > +	OPTIMIZER_HIDE_VAR(dependent_ptr_mb_val);			\
> > > > > > +	(typeof(ptr))(dependent_ptr_mb_ptr + dependent_ptr_mb_val);	\
> > > > > > +})
> > > > > > +
> > > > > > +#else
> > > > > > +
> > > > > > +#define dependent_ptr_mb(ptr, val) ({ mb(); (ptr); })
> > > > > 
> > > > > 
> > > > > So for the example of patch 4, we'd better fall back to rmb() or need a
> > > > > dependent_ptr_rmb()?
> > > > > 
> > > > > Thanks
> > > > 
> > > > You mean for strongly ordered architectures like Intel?
> > > > Yes, maybe it makes sense to have dependent_ptr_smp_rmb,
> > > > dependent_ptr_dma_rmb and dependent_ptr_virt_rmb.
> > > > 
> > > > mb variant is unused right now so I'll remove it.
> > > 
> > > How about naming the thing: dependent_ptr() ? That is without any (r)mb
> > > implications at all. The address dependency is strictly weaker than an
> > > rmb in that it will only order the two loads in qestion and not, like
> > > rmb, any prior to any later load.
> > 
> > So I'm fine with this as it's enough for virtio, but I would like to point out two things:
> > 
> > 1. E.g. on x86 both SMP and DMA variants can be NOPs but
> > the madatory one can't, so assuming we do not want
> > it to be stronger than rmp then either we want
> > smp_dependent_ptr(), dma_dependent_ptr(), dependent_ptr()
> > or we just will specify that dependent_ptr() works for
> > both DMA and SMP.
> > 
> > 2. Down the road, someone might want to order a store after a load.
> > Address dependency does that for us too. Assuming we make
> > dependent_ptr a NOP on x86, we will want an mb variant
> > which isn't a NOP on x86. Will we want to rename
> > dependent_ptr to dependent_ptr_rmb at that point?
> 
> But x86 preserves store-after-load orderings anyway, and even Alpha
> respects ordering from loads to dependent stores.  So what am I missing
> here?
> 
> 							Thanx, Paul

Oh you are right. Stores are not reordered with older loads on x86.

So point 2 is moot. Sorry about the noise.

I guess at this point the only sticking point is the ECC compiler.
I'm inclined to stick an mb() there, seeing as it doesn't even
have spectre protection enabled. Slow but safe.

-- 
MST