Re: [PATCH v3 1/8] x86: Improve __phys_addr performance by making use of carry flags and inlining

Kirill A. Shutemov
2012-11-05T20:29:06Z

On Mon, Nov 05, 2012 at 11:04:06AM -0800, Alexander Duyck wrote:
> This patch is meant to improve overall system performance when making use of
> the __phys_addr call. To do this I have implemented several changes.
>
> First if CONFIG_DEBUG_VIRTUAL is not defined __phys_addr is made an inline,
> similar to how this is currently handled in 32 bit. However in order to do
> this it is required to export phys_base so that it is available if __phys_addr
> is used in kernel modules.
>
> The second change was to streamline the code by making use of the carry flag
> on an add operation instead of performing a compare on a 64 bit value. The
> advantage to this is that it allows us to significantly reduce the overall
> size of the call. On my Xeon E5 system the entire __phys_addr inline call
> consumes a little less than 32 bytes and 5 instructions. I also applied
> similar logic to the debug version of the function. My testing shows that the
> debug version of the function with this patch applied is slightly faster than
> the non-debug version without the patch.
>
> When building the kernel with the first two changes applied I saw build
> warnings about __START_KERNEL_map and PAGE_OFFSET constants not fitting in
> their type. In order to resolve the build warning I changed their type from
> UL to ULL.

What kind of warning messages did you see?
It's strange: sizeof(unsinged long) == sizeof(unsinged long long) on
x86_64

>
> Finally I also applied the same logic changes to __virt_addr_valid since it
> used the same general code flow as __phys_addr and could achieve similar gains
> though these changes.
>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
> ---
>
> v3: Added changes to __virt_addr_valid to keep it in sync with __phys_addr
>
> arch/x86/include/asm/page_64_types.h | 17 +++++++++++++-
> arch/x86/kernel/x8664_ksyms_64.c | 3 +++
> arch/x86/mm/physaddr.c | 40 +++++++++++++++++++++-------------
> 3 files changed, 43 insertions(+), 17 deletions(-)
>
> diff --git a/arch/x86/include/asm/page_64_types.h b/arch/x86/include/asm/page_64_types.h
> index 320f7bb..1ca93d3 100644
> --- a/arch/x86/include/asm/page_64_types.h
> +++ b/arch/x86/include/asm/page_64_types.h
> @@ -30,14 +30,14 @@
> * hypervisor to fit. Choosing 16 slots here is arbitrary, but it's
> * what Xen requires.
> */
> -#define __PAGE_OFFSET _AC(0xffff880000000000, UL)
> +#define __PAGE_OFFSET _AC(0xffff880000000000, ULL)
>
> #define __PHYSICAL_START ((CONFIG_PHYSICAL_START + \
> (CONFIG_PHYSICAL_ALIGN - 1)) & \
> ~(CONFIG_PHYSICAL_ALIGN - 1))
>
> #define __START_KERNEL (__START_KERNEL_map + __PHYSICAL_START)
> -#define __START_KERNEL_map _AC(0xffffffff80000000, UL)
> +#define __START_KERNEL_map _AC(0xffffffff80000000, ULL)
>
> /* See Documentation/x86/x86_64/mm.txt for a description of the memory map. */
> #define __PHYSICAL_MASK_SHIFT 46
> @@ -58,7 +58,20 @@ void copy_page(void *to, void *from);
> extern unsigned long max_pfn;
> extern unsigned long phys_base;
>
> +static inline unsigned long __phys_addr_nodebug(unsigned long x)
> +{
> + unsigned long y = x - __START_KERNEL_map;

With change above you assign ULL const to unsigned long variable. hm?

> +
> + /* use the carry flag to determine if x was < __START_KERNEL_map */
> + x = y + ((x > y) ? phys_base : (__START_KERNEL_map - PAGE_OFFSET));
> +
> + return x;
> +}
> +#ifdef CONFIG_DEBUG_VIRTUAL
> extern unsigned long __phys_addr(unsigned long);
> +#else
> +#define __phys_addr(x) __phys_addr_nodebug(x)
> +#endif
> #define __phys_reloc_hide(x) (x)
>
> #define vmemmap ((struct page *)VMEMMAP_START)
> diff --git a/arch/x86/kernel/x8664_ksyms_64.c b/arch/x86/kernel/x8664_ksyms_64.c
> index 1330dd1..b014d94 100644
> --- a/arch/x86/kernel/x8664_ksyms_64.c
> +++ b/arch/x86/kernel/x8664_ksyms_64.c
> @@ -59,6 +59,9 @@ EXPORT_SYMBOL(memcpy);
> EXPORT_SYMBOL(__memcpy);
> EXPORT_SYMBOL(memmove);
>
> +#ifndef CONFIG_DEBUG_VIRTUAL
> +EXPORT_SYMBOL(phys_base);
> +#endif
> EXPORT_SYMBOL(empty_zero_page);
> #ifndef CONFIG_PARAVIRT
> EXPORT_SYMBOL(native_load_gs_index);
> diff --git a/arch/x86/mm/physaddr.c b/arch/x86/mm/physaddr.c
> index d2e2735..fd40d75 100644
> --- a/arch/x86/mm/physaddr.c
> +++ b/arch/x86/mm/physaddr.c
> @@ -8,33 +8,43 @@
>
> #ifdef CONFIG_X86_64
>
> +#ifdef CONFIG_DEBUG_VIRTUAL
> unsigned long __phys_addr(unsigned long x)
> {
> - if (x >= __START_KERNEL_map) {
> - x -= __START_KERNEL_map;
> - VIRTUAL_BUG_ON(x >= KERNEL_IMAGE_SIZE);
> - x += phys_base;
> + unsigned long y = x - __START_KERNEL_map;
> +
> + /* use the carry flag to determine if x was < __START_KERNEL_map */
> + if (unlikely(x > y)) {
> + x = y + phys_base;
> +
> + VIRTUAL_BUG_ON(y >= KERNEL_IMAGE_SIZE);
> } else {
> - VIRTUAL_BUG_ON(x < PAGE_OFFSET);
> - x -= PAGE_OFFSET;
> - VIRTUAL_BUG_ON(!phys_addr_valid(x));
> + x = y + (__START_KERNEL_map - PAGE_OFFSET);
> +
> + /* carry flag will be set if starting x was >= PAGE_OFFSET */
> + VIRTUAL_BUG_ON((x > y) || !phys_addr_valid(x));
> }
> +
> return x;
> }
> EXPORT_SYMBOL(__phys_addr);
> +#endif
>
> bool __virt_addr_valid(unsigned long x)
> {
> - if (x >= __START_KERNEL_map) {
> - x -= __START_KERNEL_map;
> - if (x >= KERNEL_IMAGE_SIZE)
> + unsigned long y = x - __START_KERNEL_map;
> +
> + /* use the carry flag to determine if x was < __START_KERNEL_map */
> + if (unlikely(x > y)) {
> + x = y + phys_base;
> +
> + if (y >= KERNEL_IMAGE_SIZE)
> return false;
> - x += phys_base;
> } else {
> - if (x < PAGE_OFFSET)
> - return false;
> - x -= PAGE_OFFSET;
> - if (!phys_addr_valid(x))
> + x = y + (__START_KERNEL_map - PAGE_OFFSET);
> +
> + /* carry flag will be set if starting x was >= PAGE_OFFSET */
> + if ((x > y) || !phys_addr_valid(x))
> return false;
> }
>
>
> --
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--
Kirill A. Shutemov