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 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, Alex -- 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
