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

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 2014-06-24 at 15:22 +0200, Alexander Graf wrote:
> On 24.06.14 15:01, Alexey Kardashevskiy wrote:
> > On 06/24/2014 10:52 PM, Alexander Graf wrote:
> >> 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.
> > It was like this (and this is just confusing):
> >
> > iowrite32(le32_to_cpu(val), io + off);
> >
> > What would make sense (according to you and I would understand this) is this:
> >
> > iowrite32(cpu_to_le32(val), io + off);
> >
> >
> > Or I missed your point, did I?
> 
> No, you didn't miss it. I think for people who know how iowrite32() 
> works the above is obvious. I find the fact that iowrite32() writes in 
> LE always pretty scary though ;).
> 
> So IMHO we should either create new, generic iowrite helpers that don't 
> do any endian swapping at all or do iowrite32(cpu_to_le32(val)) calls.

I'm one of those people for whom iowrite32(le32_to_cpu(val)) makes sense
and keeps the byte order consistent regardless of the platform, while
iowrite32(val) or iowrite32be(val) makes me scratch my head and try to
remember that the byte swaps are a nop on the given platforms.  As Ben
noted, a native, no-swap ioread/write doesn't exist, but perhaps should.
I'd prefer an attempt be made to make it exist before adding
vfio-specific macros.  vfio is arguably doing the right thing here given
the functions available.  Thanks,

Alex