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

[Alexander Graf]

On 24.06.14 14:50, Alexey Kardashevskiy wrote:
> On 06/24/2014 08:41 PM, Alexander Graf wrote:
> > 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?
> iowrite32 is a non-inline function so conversions take place so are the
> others. And sorry but I fail to see why this matters. We are not trying to
> accelerate things, we are removing redundant operations which confuse
> people who read the code.

The confusion depends on where you're coming from. If you happen to know 
that "iowrite32" writes in LE, then the LE conversion makes a lot of sense.

I don't have a strong feeling either way though and will let Alex decide 
on the path forward :).


Alex

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