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

From: Alexey Kardashevskiy <[email-masked]>
Date: Tue, 24 Jun 2014 22:50:35 +1000

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.

-- 
Alexey