Re: [PATCH 4/6] arm64/io: Provide a WC friendly __iowriteXX_copy()
From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2024-02-29 10:25:06
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linux-arch, linux-patches, linux-rdma, linux-s390, llvm, netdev
On Wed, Feb 28, 2024 at 07:06:16PM -0400, Jason Gunthorpe wrote:
On Tue, Feb 27, 2024 at 10:37:18AM +0000, Catalin Marinas wrote:quoted
On Tue, Feb 20, 2024 at 09:17:08PM -0400, Jason Gunthorpe wrote:quoted
+/* + * This generates a memcpy that works on a from/to address which is aligned to + * bits. Count is in terms of the number of bits sized quantities to copy. It + * optimizes to use the STR groupings when possible so that it is WC friendly. + */ +#define memcpy_toio_aligned(to, from, count, bits) \ + ({ \ + volatile u##bits __iomem *_to = to; \ + const u##bits *_from = from; \ + size_t _count = count; \ + const u##bits *_end_from = _from + ALIGN_DOWN(_count, 8); \ + \ + for (; _from < _end_from; _from += 8, _to += 8) \ + __const_memcpy_toio_aligned##bits(_to, _from, 8); \ + if ((_count % 8) >= 4) { \ + __const_memcpy_toio_aligned##bits(_to, _from, 4); \ + _from += 4; \ + _to += 4; \ + } \ + if ((_count % 4) >= 2) { \ + __const_memcpy_toio_aligned##bits(_to, _from, 2); \ + _from += 2; \ + _to += 2; \ + } \ + if (_count % 2) \ + __const_memcpy_toio_aligned##bits(_to, _from, 1); \ + })Do we actually need all this if count is not constant? If it's not performance critical anywhere, I'd rather copy the generic implementation, it's easier to read.Which generic version?
The current __iowriteXX_copy() in lib/iomap_copy.c (copy them over or add some preprocessor reuse the generic functions).
The point is to maximize WC effects with non-constant values, so I think we do need something like this. ie we can't just fall back to looping over 64 bit stores one at a time.
If that's a case you are also targeting and have seen it in practice, that's fine. But I had the impression that you are mostly after the constant count case which is already addressed by the other part of this patch. For the non-constant case, we have a DGH only at the end of whatever buffer was copied rather than after every 64-byte increments you'd get for a count of 8.
Most places I know about using this are performance paths, the entire iocopy infrastructure was introduced as an x86 performance optimization..
At least the x86 case makes sense even from a maintenance perspective, it's just a much simpler "rep movsl". I just want to make sure we don't over-complicate this code on arm64 unnecessarily. -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel