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

From: Alexander Graf <[email-masked]>
Date: Tue, 24 Jun 2014 15:22:49 +0200

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.

Alex