[PATCH v2 03/14] x86/asm: add uncached page clearing

[Ankur Arora <ankur.a.arora@oracle.com>]

Add clear_page_movnt(), which uses MOVNTI as the underlying primitive.
MOVNTI skips the memory hierarchy, so this provides a non cache-polluting
implementation of clear_page().

MOVNTI, from the Intel SDM, Volume 2B, 4-101:
 "The non-temporal hint is implemented by using a write combining (WC)
  memory type protocol when writing the data to memory. Using this
  protocol, the processor does not write the data into the cache
  hierarchy, nor does it fetch the corresponding cache line from memory
  into the cache hierarchy."

The AMD Arch Manual has something similar to say as well.

One use-case is to handle zeroing large extents where this can help by
not needlessly bring in cache-lines that would never get accessed.
Also, often clear_page_movnt() based clearing is faster once extent
sizes are O(LLC-size).

As the excerpt notes, MOVNTI is weakly ordered with respect to other
instructions operating on the memory hierarchy. This needs to be
handled by the caller by executing an SFENCE when done.

The implementation is fairly straight-forward. We unroll the inner loop
to keep it similar to memset_movnti(), so we can use that to gauge the
clear_page_movnt() performance via perf bench mem memset.

 # Intel Icelake-X
 # Performance comparison of 'perf bench mem memset -l 1' for x86-64-stosb
 # (X86_FEATURE_ERMS) and x86-64-movnt:

 System:      Oracle X9-2 (2 nodes * 32 cores * 2 threads)
 Processor:   Intel(R) Xeon(R) Platinum 8358 CPU @ 2.60GHz (Icelake-X)
 Memory:      512 GB evenly split between nodes
 LLC-size:    48MB for each node (32-cores * 2-threads)
 no_turbo: 1, Microcode: 0xd0001e0, scaling-governor: performance

              x86-64-stosb (5 runs)     x86-64-movnt (5 runs)      diff
              ----------------------    ---------------------      -------
     size            BW   (   stdev)          BW   (   stdev)

      2MB      14.37 GB/s ( +- 1.55)     12.59 GB/s ( +- 1.20)     -12.38%
     16MB      16.93 GB/s ( +- 2.61)     15.91 GB/s ( +- 2.74)      -6.02%
    128MB      12.12 GB/s ( +- 1.06)     22.33 GB/s ( +- 1.84)     +84.24%
   1024MB      12.12 GB/s ( +- 0.02)     23.92 GB/s ( +- 0.14)     +97.35%
   4096MB      12.08 GB/s ( +- 0.02)     23.98 GB/s ( +- 0.18)     +98.50%

Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 arch/x86/include/asm/page_64.h |  1 +
 arch/x86/lib/clear_page_64.S   | 26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
index 4bde0dc66100..cfb95069cf9e 100644
--- a/arch/x86/include/asm/page_64.h
+++ b/arch/x86/include/asm/page_64.h
@@ -43,6 +43,7 @@ extern unsigned long __phys_addr_symbol(unsigned long);
 void clear_page_orig(void *page);
 void clear_page_rep(void *page);
 void clear_page_erms(void *page);
+void clear_page_movnt(void *page);
 
 static inline void clear_page(void *page)
 {
diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S
index c4c7dd115953..578f40db0716 100644
--- a/arch/x86/lib/clear_page_64.S
+++ b/arch/x86/lib/clear_page_64.S
@@ -50,3 +50,29 @@ SYM_FUNC_START(clear_page_erms)
 	ret
 SYM_FUNC_END(clear_page_erms)
 EXPORT_SYMBOL_GPL(clear_page_erms)
+
+/*
+ * Zero a page.
+ * %rdi - page
+ *
+ * Caller needs to issue a sfence at the end.
+ */
+SYM_FUNC_START(clear_page_movnt)
+	xorl	%eax,%eax
+	movl	$4096,%ecx
+
+	.p2align 4
+.Lstart:
+        movnti  %rax, 0x00(%rdi)
+        movnti  %rax, 0x08(%rdi)
+        movnti  %rax, 0x10(%rdi)
+        movnti  %rax, 0x18(%rdi)
+        movnti  %rax, 0x20(%rdi)
+        movnti  %rax, 0x28(%rdi)
+        movnti  %rax, 0x30(%rdi)
+        movnti  %rax, 0x38(%rdi)
+        addq    $0x40, %rdi
+        subl    $0x40, %ecx
+        ja      .Lstart
+	ret
+SYM_FUNC_END(clear_page_movnt)
-- 
2.29.2