Re: [PATCH rdma-next 1/2] arm64/io: add memcpy_toio_64

[Robin Murphy <robin.murphy@arm.com>]

On 23/11/2023 7:04 pm, Leon Romanovsky wrote:
> From: Jason Gunthorpe <jgg@nvidia.com>
> 
> The kernel supports write combining IO memory which is commonly used to
> generate 64 byte TLPs in a PCIe environment. On many CPUs this mechanism
> is pretty tolerant and a simple C loop will suffice to generate a 64 byte
> TLP.
> 
> However modern ARM64 CPUs are quite sensitive and a compiler generated
> loop is not enough to reliably generate a 64 byte TLP. Especially given
> the ARM64 issue that writel() does not codegen anything other than "[xN]"
> as the address calculation.
> 
> These newer CPUs require an orderly consecutive block of stores to work
> reliably. This is best done with four STP integer instructions (perhaps
> ST64B in future), or a single ST4 vector instruction.
> 
> Provide a new generic function memcpy_toio_64() which should reliably
> generate the needed instructions for the architecture, assuming address
> alignment. As the usual need for this operation is performance sensitive a
> fast inline implementation is preferred.
> 
> Implement an optimized version on ARM that is a block of 4 STP
> instructions.
> 
> The generic implementation is just a simple loop. x86-64 (clang 16)
> compiles this into an unrolled loop of 16 movq pairs.
> 
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: linux-arch@vger.kernel.org
> Cc: linux-arm-kernel@lists.infradead.org
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> ---
>   arch/arm64/include/asm/io.h | 20 ++++++++++++++++++++
>   include/asm-generic/io.h    | 30 ++++++++++++++++++++++++++++++
>   2 files changed, 50 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/io.h b/arch/arm64/include/asm/io.h
> index 3b694511b98f..73ab91913790 100644
> --- a/arch/arm64/include/asm/io.h
> +++ b/arch/arm64/include/asm/io.h
> @@ -135,6 +135,26 @@ extern void __memset_io(volatile void __iomem *, int, size_t);
>   #define memcpy_fromio(a,c,l)	__memcpy_fromio((a),(c),(l))
>   #define memcpy_toio(c,a,l)	__memcpy_toio((c),(a),(l))
>   
> +static inline void __memcpy_toio_64(volatile void __iomem *to, const void *from)
> +{
> +	const u64 *from64 = from;
> +
> +	/*
> +	 * Newer ARM core have sensitive write combining buffers, it is
> +	 * important that the stores be contiguous blocks of store instructions.
> +	 * Normal memcpy does not work reliably.
> +	 */
> +	asm volatile("stp %x0, %x1, [%8, #16 * 0]\n"
> +		     "stp %x2, %x3, [%8, #16 * 1]\n"
> +		     "stp %x4, %x5, [%8, #16 * 2]\n"
> +		     "stp %x6, %x7, [%8, #16 * 3]\n"
> +		     :
> +		     : "rZ"(from64[0]), "rZ"(from64[1]), "rZ"(from64[2]),
> +		       "rZ"(from64[3]), "rZ"(from64[4]), "rZ"(from64[5]),
> +		       "rZ"(from64[6]), "rZ"(from64[7]), "r"(to));

Is this correct for big-endian? LDP/STP are kinda tricksy in that regard.

Thanks,
Robin.

> +}
> +#define memcpy_toio_64(to, from) __memcpy_toio_64(to, from)
> +
>   /*
>    * I/O memory mapping functions.
>    */
> diff --git a/include/asm-generic/io.h b/include/asm-generic/io.h
> index bac63e874c7b..2d6d60ed2128 100644
> --- a/include/asm-generic/io.h
> +++ b/include/asm-generic/io.h
> @@ -1202,6 +1202,36 @@ static inline void memcpy_toio(volatile void __iomem *addr, const void *buffer,
>   }
>   #endif
>   
> +#ifndef memcpy_toio_64
> +#define memcpy_toio_64 memcpy_toio_64
> +/**
> + * memcpy_toio_64	Copy 64 bytes of data into I/O memory
> + * @dst:		The (I/O memory) destination for the copy
> + * @src:		The (RAM) source for the data
> + * @count:		The number of bytes to copy
> + *
> + * dst and src must be aligned to 8 bytes. This operation copies exactly 64
> + * bytes. It is intended to be used for write combining IO memory. The
> + * architecture should provide an implementation that has a high chance of
> + * generating a single combined transaction.
> + */
> +static inline void memcpy_toio_64(volatile void __iomem *addr,
> +				  const void *buffer)
> +{
> +	unsigned int i = 0;
> +
> +#if BITS_PER_LONG == 64
> +	for (; i != 8; i++)
> +		__raw_writeq(((const u64 *)buffer)[i],
> +			     ((u64 __iomem *)addr) + i);
> +#else
> +	for (; i != 16; i++)
> +		__raw_writel(((const u32 *)buffer)[i],
> +			     ((u32 __iomem *)addr) + i);
> +#endif
> +}
> +#endif
> +
>   extern int devmem_is_allowed(unsigned long pfn);
>   
>   #endif /* __KERNEL__ */