Re: [Xen-devel] [PATCH 2/2] xen: arm: update arm32 assembly primitives to Linux v3.16-rc6

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2014-07-25 at 17:03 +0100, Ian Campbell wrote:
> On Fri, 2014-07-25 at 16:48 +0100, Julien Grall wrote:
> > On 07/25/2014 04:48 PM, Ian Campbell wrote:
> > > On Fri, 2014-07-25 at 16:42 +0100, Julien Grall wrote:
> > >> Hi Ian,
> > >>
> > >> On 07/25/2014 04:22 PM, Ian Campbell wrote:
> > >>> bitops, cmpxchg, atomics: Import:
> > >>>   c32ffce ARM: 7984/1: prefetch: add prefetchw invocations for 
> > >>> barriered atomics
> > >>
> > >> Compare to Linux we don't have specific prefetch* helpers. We directly
> > >> use the compiler builtin ones. Shouldn't we import the ARM specific
> > >> helpers to gain in performance?
> > > 
> > > My binaries are full of pld instructions where I think I would expect
> > > them, so it seems like the compiler builtin ones are sufficient.
> > > 
> > > I suspect the Linux define is there to cope with older compilers or
> > > something.
> > 
> > If so:
> 
> The compiled output is very different if I use the arch specific
> explicit variants. The explicit variant generates (lots) more pldw and
> (somewhat) fewer pld. I've no idea what this means...

It's a bit more obvious for aarch64 where gcc 4.8 doesn't generate any
prefetches at all via the builtins...

Here's what I've got in my tree. I've no idea if we should take some or
all of it...

Ian.

8<-----------------

From feb516fee01a0af60f54337b323975154eb466d8 Mon Sep 17 00:00:00 2001
Message-Id: 
<feb516fee01a0af60f54337b323975154eb466d8.1406304807.git.ian.campbell@xxxxxxxxxx>
From: Ian Campbell <ian.campbell@xxxxxxxxxx>
Date: Fri, 25 Jul 2014 17:08:42 +0100
Subject: [PATCH] xen: arm: Use explicit prefetch instructions.

On ARM32 these certainly generate *different* sets of prefetches.
I've no clue if that is a good thing...

On ARM64 the builtin variants seems to be non-functional (at least
with gcc 4.8).

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
---
 xen/include/asm-arm/arm32/processor.h |   17 +++++++++++++++++
 xen/include/asm-arm/arm64/processor.h |   22 ++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/xen/include/asm-arm/arm32/processor.h 
b/xen/include/asm-arm/arm32/processor.h
index f41644d..6feacc9 100644
--- a/xen/include/asm-arm/arm32/processor.h
+++ b/xen/include/asm-arm/arm32/processor.h
@@ -119,6 +119,23 @@ struct cpu_user_regs
 #define cpu_has_erratum_766422()                             \
     (unlikely(current_cpu_data.midr.bits == 0x410fc0f4))
 
+#define ARCH_HAS_PREFETCH
+static inline void prefetch(const void *ptr)
+{
+        __asm__ __volatile__(
+                "pld\t%a0"
+                :: "p" (ptr));
+}
+
+#define ARCH_HAS_PREFETCHW
+static inline void prefetchw(const void *ptr)
+{
+        __asm__ __volatile__(
+                ".arch_extension        mp\n"
+                "pldw\t%a0"
+                :: "p" (ptr));
+}
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* __ASM_ARM_ARM32_PROCESSOR_H */
diff --git a/xen/include/asm-arm/arm64/processor.h 
b/xen/include/asm-arm/arm64/processor.h
index 5bf0867..56b1002 100644
--- a/xen/include/asm-arm/arm64/processor.h
+++ b/xen/include/asm-arm/arm64/processor.h
@@ -106,6 +106,28 @@ struct cpu_user_regs
 
 #define cpu_has_erratum_766422() 0
 
+/*
+ * Prefetching support
+ */
+#define ARCH_HAS_PREFETCH
+static inline void prefetch(const void *ptr)
+{
+        asm volatile("prfm pldl1keep, %a0\n" : : "p" (ptr));
+}
+
+#define ARCH_HAS_PREFETCHW
+static inline void prefetchw(const void *ptr)
+{
+        asm volatile("prfm pstl1keep, %a0\n" : : "p" (ptr));
+}
+
+#define ARCH_HAS_SPINLOCK_PREFETCH
+static inline void spin_lock_prefetch(const void *x)
+{
+        prefetchw(x);
+}
+
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* __ASM_ARM_ARM64_PROCESSOR_H */
-- 
1.7.10.4




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