Re: [PATCH] vfio: Fix endianness handling for emulated BARs

[Alexander Graf]

On 24.06.14 12:11, Alexey Kardashevskiy wrote:
> On 06/21/2014 09:12 AM, Benjamin Herrenschmidt wrote:
> > On Thu, 2014-06-19 at 21:21 -0600, Alex Williamson wrote:
> >
> > > Working on big endian being an accident may be a matter of perspective
> >   :-)
> >
> > > The comment remains that this patch doesn't actually fix anything except
> > > the overhead on big endian systems doing redundant byte swapping and
> > > maybe the philosophy that vfio regions are little endian.
> > Yes, that works by accident because technically VFIO is a transport and
> > thus shouldn't perform any endian swapping of any sort, which remains
> > the responsibility of the end driver which is the only one to know
> > whether a given BAR location is a a register or some streaming data
> > and in the former case whether it's LE or BE (some PCI devices are BE
> > even ! :-)
> >
> > But yes, in the end, it works with the dual "cancelling" swaps and the
> > overhead of those swaps is probably drowned in the noise of the syscall
> > overhead.
> >
> > > I'm still not a fan of iowrite vs iowritebe, there must be something we
> > > can use that doesn't have an implicit swap.
> > Sadly there isn't ... In the old day we didn't even have the "be"
> > variant and readl/writel style accessors still don't have them either
> > for all archs.
> >
> > There is __raw_readl/writel but here the semantics are much more than
> > just "don't swap", they also don't have memory barriers (which means
> > they are essentially useless to most drivers unless those are platform
> > specific drivers which know exactly what they are doing, or in the rare
> > cases such as accessing a framebuffer which we know never have side
> > effects).
> >
> > >   Calling it iowrite*_native is also an abuse of the namespace.
> >
> > >   Next thing we know some common code
> > > will legitimately use that name.
> > I might make sense to those definitions into a common header. There have
> > been a handful of cases in the past that wanted that sort of "native
> > byte order" MMIOs iirc (though don't ask me for examples, I can't really
> > remember).
> >
> > >   If we do need to define an alias
> > > (which I'd like to avoid) it should be something like vfio_iowrite32.
>
> Ping?
>
> We need to make a decision whether to move those xxx_native() helpers
> somewhere (where?) or leave the patch as is (as we figured out that
> iowriteXX functions implement barriers and we cannot just use raw
> accessors) and fix commit log to explain everything.

Is there actually any difference in generated code with this patch 
applied and without? I would hope that iowrite..() is inlined and 
cancels out the cpu_to_le..() calls that are also inlined?


Alex

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