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

From: Alexey Kardashevskiy <[email-masked]>
Date: Tue, 24 Jun 2014 23:01:43 +1000

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?

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

It would probably help if you picked the side :)

-- 
Alexey