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[PATCH 2/2] x86, boot: Use regparm=0 for memcpy and memset when using clang

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  [PATCH 2/2] x86, boot: Use regparm=0 for memcpy and memset when using clang Michael Davidson <md@google.com> - 2017-03-15 22:50 +0100
    Re: [PATCH 2/2] x86, boot: Use regparm=0 for memcpy and memset when  using clang Ingo Molnar <mingo@kernel.org> - 2017-03-16 08:50 +0100
      Re: [PATCH 2/2] x86, boot: Use regparm=0 for memcpy and memset when  using clang Michael Davidson <md@google.com> - 2017-03-16 16:30 +0100

#1601760 — [PATCH 2/2] x86, boot: Use regparm=0 for memcpy and memset when using clang

Use the standard regparm=0 calling convention for memcpy and
memset when building with clang.

This is a work around for a long standing clang bug
(see https://llvm.org/bugs/show_bug.cgi?id=3997) where
clang always uses the standard regparm=0 calling convention
for any implcit calls to memcpy and memset that it generates
(eg for structure assignments and initialization) even if an
alternate calling convention such as regparm=3 has been specified.

Signed-off-by: Michael Davidson <md@google.com>
---
 arch/x86/boot/copy.S   | 15 +++++++++++++--
 arch/x86/boot/string.h | 13 +++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/copy.S b/arch/x86/boot/copy.S
index 1eb7d298b47d..57142d1ad0d2 100644
--- a/arch/x86/boot/copy.S
+++ b/arch/x86/boot/copy.S
@@ -18,6 +18,12 @@
 	.text
 
 GLOBAL(memcpy)
+#ifdef	__clang__	/* Use normal ABI calling conventions */
+	movw	4(%esp), %ax
+	movw	8(%esp), %dx
+	movw	12(%esp), %cx
+#endif
+_memcpy:
 	pushw	%si
 	pushw	%di
 	movw	%ax, %di
@@ -34,6 +40,11 @@ GLOBAL(memcpy)
 ENDPROC(memcpy)
 
 GLOBAL(memset)
+#ifdef	__clang__	/* Use normal ABI calling conventions */
+	movw	4(%esp), %ax
+	movw	8(%esp), %dx
+	movw	12(%esp), %cx
+#endif
 	pushw	%di
 	movw	%ax, %di
 	movzbl	%dl, %eax
@@ -52,7 +63,7 @@ GLOBAL(copy_from_fs)
 	pushw	%ds
 	pushw	%fs
 	popw	%ds
-	calll	memcpy
+	calll	_memcpy
 	popw	%ds
 	retl
 ENDPROC(copy_from_fs)
@@ -61,7 +72,7 @@ GLOBAL(copy_to_fs)
 	pushw	%es
 	pushw	%fs
 	popw	%es
-	calll	memcpy
+	calll	_memcpy
 	popw	%es
 	retl
 ENDPROC(copy_to_fs)
diff --git a/arch/x86/boot/string.h b/arch/x86/boot/string.h
index 113588ddb43f..e735cccb3fc8 100644
--- a/arch/x86/boot/string.h
+++ b/arch/x86/boot/string.h
@@ -6,8 +6,21 @@
 #undef memset
 #undef memcmp
 
+/*
+ * Use normal ABI calling conventions - i.e. regparm(0) -
+ * for memcpy() and memset() if we are building the real
+ * mode setup code with clang since clang may make implicit
+ * calls to these functions that assume regparm(0).
+ */
+#if defined(_SETUP) && defined(__clang__)
+void __attribute__((regparm(0))) *memcpy(void *dst, const void *src,
+					 size_t len);
+void __attribute__((regparm(0))) *memset(void *dst, int c, size_t len);
+#else
 void *memcpy(void *dst, const void *src, size_t len);
 void *memset(void *dst, int c, size_t len);
+#endif
+
 int memcmp(const void *s1, const void *s2, size_t len);
 
 /*
-- 
2.12.0.367.g23dc2f6d3c-goog

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#1602039 — Re: [PATCH 2/2] x86, boot: Use regparm=0 for memcpy and memset when using clang

* Michael Davidson <md@google.com> wrote:

> Use the standard regparm=0 calling convention for memcpy and
> memset when building with clang.
> 
> This is a work around for a long standing clang bug
> (see https://llvm.org/bugs/show_bug.cgi?id=3997) where
> clang always uses the standard regparm=0 calling convention
> for any implcit calls to memcpy and memset that it generates
> (eg for structure assignments and initialization) even if an
> alternate calling convention such as regparm=3 has been specified.
> 
> Signed-off-by: Michael Davidson <md@google.com>
> ---
>  arch/x86/boot/copy.S   | 15 +++++++++++++--
>  arch/x86/boot/string.h | 13 +++++++++++++
>  2 files changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/boot/copy.S b/arch/x86/boot/copy.S
> index 1eb7d298b47d..57142d1ad0d2 100644
> --- a/arch/x86/boot/copy.S
> +++ b/arch/x86/boot/copy.S
> @@ -18,6 +18,12 @@
>  	.text
>  
>  GLOBAL(memcpy)
> +#ifdef	__clang__	/* Use normal ABI calling conventions */
> +	movw	4(%esp), %ax
> +	movw	8(%esp), %dx
> +	movw	12(%esp), %cx
> +#endif
> +_memcpy:
>  	pushw	%si
>  	pushw	%di
>  	movw	%ax, %di
> @@ -34,6 +40,11 @@ GLOBAL(memcpy)
>  ENDPROC(memcpy)
>  
>  GLOBAL(memset)
> +#ifdef	__clang__	/* Use normal ABI calling conventions */
> +	movw	4(%esp), %ax
> +	movw	8(%esp), %dx
> +	movw	12(%esp), %cx
> +#endif

I don't think we want to add such ugly workarounds for clang bugs in a piecemail 
wise manner. If this was the _only_ workaround required to have a working kernel 
built with clang then maybe, but there's no way to tell from this submission.

Thanks,

	Ingo

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#1602528 — Re: [PATCH 2/2] x86, boot: Use regparm=0 for memcpy and memset when using clang

On Thu, Mar 16, 2017 at 12:46 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> I don't think we want to add such ugly workarounds for clang bugs in a piecemail
> wise manner. If this was the _only_ workaround required to have a working kernel
> built with clang then maybe, but there's no way to tell from this submission.
>
Apologies for the lack of context. This change, and the #undef of mem*
in x86/boot/string.c
are actually the only patches that are absolutely needed to get an
x86_64 kernel to
build and boot successfully with clang provided that you also arrange
for  clang
be invoked with the -no-integrated-as option. In practice it is also
necessary to disable
a few more clang warnings to make the build tolerable.

I will send out a more complete set of patches later today which deal
with those other
issues.

As far as the specific issue of this particular clang bug is concerned
I did also consider
adding a configuration option to simply not build the real mode boot
code at all but
that looked like it would require much more extensive surgery.

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