Re: [PATCH] arch64: optimize __memcpy_fromio, __memcpy_toio and __memset_io

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

Hi Mark,

On 20/10/17 21:22, Mark Salyzyn wrote:
> __memcpy_fromio and __memcpy_toio functions do not deal well with
> harmonically unaligned addresses unless they can ultimately be
> copied as quads (u64) to and from the destination.  Without a
> harmonically aligned relationship, they perform byte operations
> over the entire buffer.
> 
> Added optional paths that perform reads and writes at the best
> alignment possible with source and destination, placing a priority
> on using quads (8 byte transfers) on the io-side.
> 
> Removed the volatile on the source for __memcpy_toio as it is
> unnecessary.
> 
> This change was motivated by performance issues in the pstore driver.
> On a test platform, measuring probe time for pstore, console buffer
> size of 1/4MB and pmsg of 1/2MB, was in the 90-107ms region. Change
> managed to reduce it to worst case 15ms, an improvement in boot time.

Is ~90ms really worth this level of complexity? My hunch is that just
avoiding the pathological large-byte-copy case accounts for most of the
benefit, and optimisation beyond that has severely diminishing returns.

> Adjusted __memset_io to use the same pattern of access, although it
> does not have a harmonic relationship between two pointers to worry
> about, and thus the benefit is balance and not nearly as dramatic.
> 
> Signed-off-by: Mark Salyzyn <salyzyn@android.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Anton Vorontsov <anton@enomsg.org>
> Cc: Tony Luck <tony.luck@intel.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Anton Vorontsov <anton@enomsg.org>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> 
> ---
>  arch/arm64/kernel/io.c | 199 ++++++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 156 insertions(+), 43 deletions(-)
> 
> diff --git a/arch/arm64/kernel/io.c b/arch/arm64/kernel/io.c
> index 354be2a872ae..14ef7c8f20ea 100644
> --- a/arch/arm64/kernel/io.c
> +++ b/arch/arm64/kernel/io.c
> @@ -20,61 +20,147 @@
>  #include <linux/types.h>
>  #include <linux/io.h>
>  
> +/* if/while helpers assume from, to and count vars accessible in caller */
> +
> +/* arguments to helpers to ensure proper combinations */
> +#define	byte		b, u8
> +#define	word		w, u16
> +#define	longword	l, u32
> +#define	quad		q, u64
> +
> +/* read helper for unaligned transfers needing intermediate hold and memcpy */
> +#define	_do_unaligned_read(op, align_type, width, type) do {	\
> +	op(count >= sizeof(type)) {				\
> +		type hold = __raw_read##width##(from);		\
> +								\
> +		memcpy((align_type *)to, &hold, sizeof(type));	\
> +		to += sizeof(type);				\
> +		from += sizeof(type);				\
> +		count -= sizeof(type);				\
> +	}							\
> +} while (0)
> +#define if_unaligned_read(type, x) _do_unaligned_read(if, type, x)
> +#define while_unaligned_read(type, x) _do_unaligned_read(while, type, x)
> +
> +/* read helper for aligned transfers */
> +#define	_do_aligned_read(op, width, type) \
> +	_do_unaligned_read(op, type, width, type)
> +#define if_aligned_read(x) _do_aligned_read(if, x)
> +#define while_aligned_read(x) _do_aligned_read(while, x)

Yuck. That's an unreadable code construction kit if ever I saw one.

> +
>  /*
>   * Copy data from IO memory space to "real" memory space.
>   */
> +
>  void __memcpy_fromio(void *to, const volatile void __iomem *from, size_t count)
>  {
> -	while (count && (!IS_ALIGNED((unsigned long)from, 8) ||
> -			 !IS_ALIGNED((unsigned long)to, 8))) {

AFAICS, just getting rid of this one line should suffice - we shouldn't
need to care about the alignment of the destination pointer since normal
memory can handle unaligned Dword stores with mostly no penalty. This
appears to have been inherited from PowerPC without any obvious
justification.

> -		*(u8 *)to = __raw_readb(from);
> -		from++;
> -		to++;
> -		count--;
> -	}
> +	if (!IS_ALIGNED((unsigned long)from, sizeof(u16)))
> +		if_aligned_read(byte);
>  
> -	while (count >= 8) {
> -		*(u64 *)to = __raw_readq(from);
> -		from += 8;
> -		to += 8;
> -		count -= 8;
> +	if (!IS_ALIGNED((unsigned long)to, sizeof(u16))) {
> +		if (!IS_ALIGNED((unsigned long)from, sizeof(u32)))
> +			if_unaligned_read(u8, word);
> +		if (!IS_ALIGNED((unsigned long)from, sizeof(u64)))
> +			if_unaligned_read(u8, longword);
> +		while_unaligned_read(u8, quad);
> +		if_unaligned_read(u8, longword);
> +		if_unaligned_read(u8, word);
> +		if_aligned_read(byte);
> +		return;

Yup, I have absolutely no idea what that does. The fact that it returns
early implies that we have an explosion of duplicated code below,
though. I can't help wondering whether the I-cache and BTB footprint of
this bad boy ends up canceling out much of the gain from reduced
load/store bandwidth.

>  	}
>  
> -	while (count) {
> -		*(u8 *)to = __raw_readb(from);
> -		from++;
> -		to++;
> -		count--;
> +	if (!IS_ALIGNED((unsigned long)from, sizeof(u32)))
> +		if_aligned_read(word);
> +
> +	if (!IS_ALIGNED((unsigned long)to, sizeof(u32))) {
> +		if (!IS_ALIGNED((unsigned long)from, sizeof(u64)))
> +			if_unaligned_read(u16, longword);
> +		while_unaligned_read(u16, quad);
> +		if_unaligned_read(u16, longword);
> +		if_aligned_read(word);
> +		if_aligned_read(byte);
> +		return;
>  	}
> +
> +	if (!IS_ALIGNED((unsigned long)from, sizeof(u64)))
> +		if_aligned_read(longword);
> +
> +	if (!IS_ALIGNED((unsigned long)to, sizeof(u64)))
> +		while_unaligned_read(u32, quad);
> +	else
> +		while_aligned_read(quad);
> +
> +	if_aligned_read(longword);
> +	if_aligned_read(word);
> +	if_aligned_read(byte);
>  }
>  EXPORT_SYMBOL(__memcpy_fromio);
>  
> +/* write helper for unaligned transfers needing intermediate hold and memcpy */
> +#define	_do_unaligned_write(op, align_type, width, type) do {	\
> +	op(count >= sizeof(type)) {				\
> +		type hold;					\
> +								\
> +		memcpy(&hold, (align_type *)from, sizeof(type));\
> +		__raw_write##width##(hold, to);			\
> +		to += sizeof(type);				\
> +		from += sizeof(type);				\
> +		count -= sizeof(type);				\
> +	}							\
> +} while (0)
> +#define if_unaligned_write(type, x) _do_unaligned_write(if, type, x)
> +#define while_unaligned_write(type, x) _do_unaligned_write(while, type, x)
> +
> +/* write helper for aligned transfers */
> +#define	_do_aligned_write(op, width, type) \
> +	_do_unaligned_write(op, type, width, type)
> +#define if_aligned_write(x) _do_aligned_write(if, x)
> +#define while_aligned_write(x) _do_aligned_write(while, x)
> +
>  /*
>   * Copy data from "real" memory space to IO memory space.
>   */
>  void __memcpy_toio(volatile void __iomem *to, const void *from, size_t count)
>  {
> -	while (count && (!IS_ALIGNED((unsigned long)to, 8) ||
> -			 !IS_ALIGNED((unsigned long)from, 8))) {

Similarly here. We're cool with unaligned Dword loads, so aligning the
iomem pointer should be enough.

> -		__raw_writeb(*(volatile u8 *)from, to);
> -		from++;
> -		to++;
> -		count--;
> -	}
> +	if (!IS_ALIGNED((unsigned long)to, sizeof(u16)))
> +		if_aligned_write(byte);
>  
> -	while (count >= 8) {
> -		__raw_writeq(*(volatile u64 *)from, to);
> -		from += 8;
> -		to += 8;
> -		count -= 8;
> +	if (!IS_ALIGNED((unsigned long)from, sizeof(u16))) {
> +		if (!IS_ALIGNED((unsigned long)to, sizeof(u32)))
> +			if_unaligned_write(u8, word);
> +		if (!IS_ALIGNED((unsigned long)to, sizeof(u64)))
> +			if_unaligned_write(u8, longword);
> +		while_unaligned_write(u8, quad);
> +		if_unaligned_write(u8, longword);
> +		if_unaligned_write(u8, word);
> +		if_aligned_write(byte);
> +		return;
>  	}
>  
> -	while (count) {
> -		__raw_writeb(*(volatile u8 *)from, to);
> -		from++;
> -		to++;
> -		count--;
> +	if (!IS_ALIGNED((unsigned long)to, sizeof(u32)))
> +		if_aligned_write(word);
> +
> +	if (!IS_ALIGNED((unsigned long)from, sizeof(u32))) {
> +		if (!IS_ALIGNED((unsigned long)to, sizeof(u64)))
> +			if_unaligned_write(u16, longword);
> +		while_unaligned_write(u16, quad);
> +		if_unaligned_write(u16, longword);
> +		if_aligned_write(word);
> +		if_aligned_write(byte);
> +		return;
>  	}
> +
> +	if (!IS_ALIGNED((unsigned long)to, sizeof(u64)))
> +		if_aligned_write(longword);
> +
> +	if (!IS_ALIGNED((unsigned long)from, sizeof(u64)))
> +		while_unaligned_write(u32, quad);
> +	else
> +		while_aligned_write(quad);
> +
> +	if_aligned_write(longword);
> +	if_aligned_write(word);
> +	if_aligned_write(byte);
>  }
>  EXPORT_SYMBOL(__memcpy_toio);
>  
> @@ -89,22 +175,49 @@ void __memset_io(volatile void __iomem *dst, int c, size_t count)
>  	qc |= qc << 16;
>  	qc |= qc << 32;
>  
> -	while (count && !IS_ALIGNED((unsigned long)dst, 8)) {
> +	if ((count >= sizeof(u8)) &&
> +	    !IS_ALIGNED((unsigned long)dst, sizeof(u16))) {
>  		__raw_writeb(c, dst);
> -		dst++;
> -		count--;
> +		dst += sizeof(u8);
> +		count -= sizeof(u8);
> +	}
> +
> +	if ((count >= sizeof(u16)) &&
> +	    !IS_ALIGNED((unsigned long)dst, sizeof(u32))) {
> +		__raw_writew((u16)qc, dst);
> +		dst += sizeof(u16);
> +		count -= sizeof(u16);
>  	}
>  
> -	while (count >= 8) {
> +	if ((count >= sizeof(u32)) &&
> +	    !IS_ALIGNED((unsigned long)dst, sizeof(u64))) {
> +		__raw_writel((u32)qc, dst);
> +		dst += sizeof(u32);
> +		count -= sizeof(u32);
> +	}
> +
> +	while (count >= sizeof(u64)) {
>  		__raw_writeq(qc, dst);
> -		dst += 8;
> -		count -= 8;
> +		dst += sizeof(u64);
> +		count -= sizeof(u64);
> +	}
> +
> +	if (count >= sizeof(u32)) {
> +		__raw_writel((u32)qc, dst);
> +		dst += sizeof(u32);
> +		count -= sizeof(u32);
> +	}
> +
> +	if (count >= sizeof(u16)) {
> +		__raw_writew((u16)qc, dst);
> +		dst += sizeof(u16);
> +		count -= sizeof(u16);
>  	}
>  
> -	while (count) {
> +	if (count) {
>  		__raw_writeb(c, dst);
> -		dst++;
> -		count--;
> +		dst += sizeof(u8);
> +		count -= sizeof(u8);
>  	}

In the absolute worst case, this saves all of 8 iomem accesses, and
given how few callers of memset_io() there are anyway it really doesn't
seem worth the bother.

Robin.

>  }
>  EXPORT_SYMBOL(__memset_io);
>