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. > > 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, > > -- 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
