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

From: Alexander Graf <[email-masked]>
Date: Tue, 24 Jun 2014 14:52:22 +0200

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