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

From: Alexey Kardashevskiy <[email-masked]>
Date: Wed, 25 Jun 2014 00:33:28 +1000

On 06/25/2014 12:21 AM, Alex Williamson wrote:
> 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

I do not understand why @val is considered LE here and need to be converted
to CPU. Really. I truly believe it should be cpu_to_le32().

> 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,

-- 
Alexey