linux-kernel - Re: [PATCH] vfio: Fix endianness handling for emulated BARs

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Date:	Tue, 24 Jun 2014 12:41:57 +0200
From:	Alexander Graf <agraf@...e.de>
To:	Alexey Kardashevskiy <aik@...abs.ru>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Alex Williamson <alex.williamson@...hat.com>
CC:	linuxppc-dev@...ts.ozlabs.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>
Subject: Re: [PATCH] vfio: Fix endianness handling for emulated BARs


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?


Alex

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