On 06/25/2014 12:43 AM, Alex Williamson wrote: > On Wed, 2014-06-25 at 00:33 +1000, Alexey Kardashevskiy wrote: >> 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(). > > Because iowrite32 is defined to take a cpu byte order value and write it > as little endian. Ok, then neither le32_to_cpu() nor cpu_to_le32() should be there at all, if we are talking about not scratching anyone's head :) >>> 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, I do not mind to make that atempt but what exactly would make sense here? Try moving macros to include/asm-generic/io.h? Something else? Thanks. -- Alexey -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
