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 -- 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
