[PATCH] Align the scratch allocation to 64 bytes

[PATCH] Align the scratch allocation to 64 bytes

[Chao-ying Fu]

We increase the scratch allocation alignment to 64 bytes
as the cache line size, to avoid two atomic variables from
the same cache line that may cause livelock on some platforms.
---
 lib/sbi/sbi_scratch.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/lib/sbi/sbi_scratch.c b/lib/sbi/sbi_scratch.c
index ccbbc68..6b161bc 100644
--- a/lib/sbi/sbi_scratch.c
+++ b/lib/sbi/sbi_scratch.c
@@ -14,6 +14,9 @@
 #include <sbi/sbi_scratch.h>
 #include <sbi/sbi_string.h>
 
+/* Minimum size and alignment of scratch allocations */
+#define SCRATCH_ALLOC_ALIGN 64
+
 u32 last_hartindex_having_scratch = 0;
 u32 hartindex_to_hartid_table[SBI_HARTMASK_MAX_BITS + 1] = { -1U };
 struct sbi_scratch *hartindex_to_scratch_table[SBI_HARTMASK_MAX_BITS + 1] = { 0 };
@@ -70,8 +73,12 @@ unsigned long sbi_scratch_alloc_offset(unsigned long size)
 	if (!size)
 		return 0;
 
-	size += __SIZEOF_POINTER__ - 1;
-	size &= ~((unsigned long)__SIZEOF_POINTER__ - 1);
+	/*
+	 * We let the allocation align to 64 bytes to avoid livelock on
+	 * certain platforms due to atomic variables from the same cache line.
+	 */
+	size += SCRATCH_ALLOC_ALIGN - 1;
+	size &= ~((unsigned long)SCRATCH_ALLOC_ALIGN - 1);
 
 	spin_lock(&extra_lock);
 
-- 
2.47.1

[PATCH] Align the scratch allocation to 64 bytes

[Xiang W]

? 2025-01-09?? 16:34 -0800?Chao-ying Fu???
> We increase the scratch allocation alignment to 64 bytes
> as the cache line size, to avoid two atomic variables from
> the same cache line that may cause livelock on some platforms.
> ---
> ?lib/sbi/sbi_scratch.c | 11 +++++++++--
> ?1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/sbi/sbi_scratch.c b/lib/sbi/sbi_scratch.c
> index ccbbc68..6b161bc 100644
> --- a/lib/sbi/sbi_scratch.c
> +++ b/lib/sbi/sbi_scratch.c
> @@ -14,6 +14,9 @@
> ?#include <sbi/sbi_scratch.h>
> ?#include <sbi/sbi_string.h>
> ?
> +/* Minimum size and alignment of scratch allocations */
> +#define SCRATCH_ALLOC_ALIGN 64
This appears to be a platform-specific need and should not affect all platforms.
Can be like this:

#ifndef SCRATCH_ALLOC_ALIGN
#define SCRATCH_ALLOC_ALIGN __SIZEOF_POINTER__
#endif

Regards,
Xiang W
> +
> ?u32 last_hartindex_having_scratch = 0;
> ?u32 hartindex_to_hartid_table[SBI_HARTMASK_MAX_BITS + 1] = { -1U };
> ?struct sbi_scratch *hartindex_to_scratch_table[SBI_HARTMASK_MAX_BITS + 1] = { 0 };
> @@ -70,8 +73,12 @@ unsigned long sbi_scratch_alloc_offset(unsigned long size)
> ?	if (!size)
> ?		return 0;
> ?
> -	size += __SIZEOF_POINTER__ - 1;
> -	size &= ~((unsigned long)__SIZEOF_POINTER__ - 1);
> +	/*
> +	 * We let the allocation align to 64 bytes to avoid livelock on
> +	 * certain platforms due to atomic variables from the same cache line.
> +	 */
> +	size += SCRATCH_ALLOC_ALIGN - 1;
> +	size &= ~((unsigned long)SCRATCH_ALLOC_ALIGN - 1);
> ?
> ?	spin_lock(&extra_lock);
> ?
> -- 
> 2.47.1
> 
> 

[PATCH] Align the scratch allocation to 64 bytes

[Chao-ying Fu]

On Thu, Jan 9, 2025 at 8:15?PM Xiang W <wxjstz@126.com> wrote:
>
> ? 2025-01-09?? 16:34 -0800?Chao-ying Fu???
> > We increase the scratch allocation alignment to 64 bytes
> > as the cache line size, to avoid two atomic variables from
> > the same cache line that may cause livelock on some platforms.
> > ---
> >  lib/sbi/sbi_scratch.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/lib/sbi/sbi_scratch.c b/lib/sbi/sbi_scratch.c
> > index ccbbc68..6b161bc 100644
> > --- a/lib/sbi/sbi_scratch.c
> > +++ b/lib/sbi/sbi_scratch.c
> > @@ -14,6 +14,9 @@
> >  #include <sbi/sbi_scratch.h>
> >  #include <sbi/sbi_string.h>
> >
> > +/* Minimum size and alignment of scratch allocations */
> > +#define SCRATCH_ALLOC_ALIGN 64
> This appears to be a platform-specific need and should not affect all platforms.
> Can be like this:
>
> #ifndef SCRATCH_ALLOC_ALIGN
> #define SCRATCH_ALLOC_ALIGN __SIZEOF_POINTER__
> #endif
>
> Regards,
> Xiang W

Yes, I will revise the patch to use the original alignment, if
SCRATCH_ALLOC_ALIGN is not defined.
Thanks a lot!

Regards,
Chao-ying

[PATCH v2] We add a new configuration CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT as an int

[Raj Vishwanathan]

If it is set to 0 or not defined, we will continue with the previous
defintion of allocating pointer size chunks. Otherwise we will use the
chacheline size of 64.
We have the option of increasing the scratch allocation alignment to 64 bytes
as the cache line size, to avoid two atomic variables from
the same cache line that may cause livelock on some platforms.
---
 lib/sbi/Kconfig       |  8 ++++++++
 lib/sbi/sbi_scratch.c | 18 ++++++++++++++++--
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/lib/sbi/Kconfig b/lib/sbi/Kconfig
index c6cc04b..1ffa170 100644
--- a/lib/sbi/Kconfig
+++ b/lib/sbi/Kconfig
@@ -69,4 +69,12 @@ config SBI_ECALL_SSE
 config SBI_ECALL_MPXY
 	bool "MPXY extension"
 	default y
+config SBI_SCRATCH_ALLOC_ALIGNMENT
+	int  "Scratch allocation alignment"
+	default 0
+	help
+	  We provide the option for allocation alignment to 64 bytes to avoid 
+	  livelock on certain platforms due to atomic variables from the same 
+	  cache line.  Leave it 0 for default behaviour.
+
 endmenu
diff --git a/lib/sbi/sbi_scratch.c b/lib/sbi/sbi_scratch.c
index ccbbc68..88ea3c7 100644
--- a/lib/sbi/sbi_scratch.c
+++ b/lib/sbi/sbi_scratch.c
@@ -14,6 +14,13 @@
 #include <sbi/sbi_scratch.h>
 #include <sbi/sbi_string.h>
 
+#if !defined(CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT) || (CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT==0)
+#define SCRATCH_ALLOC_ALIGNMENT __SIZEOF_POINTER__
+#else
+#define SCRATCH_ALLOC_ALIGNMENT CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT
+#endif
+
+
 u32 last_hartindex_having_scratch = 0;
 u32 hartindex_to_hartid_table[SBI_HARTMASK_MAX_BITS + 1] = { -1U };
 struct sbi_scratch *hartindex_to_scratch_table[SBI_HARTMASK_MAX_BITS + 1] = { 0 };
@@ -70,8 +77,15 @@ unsigned long sbi_scratch_alloc_offset(unsigned long size)
 	if (!size)
 		return 0;
 
-	size += __SIZEOF_POINTER__ - 1;
-	size &= ~((unsigned long)__SIZEOF_POINTER__ - 1);
+	/*
+	 * We let the allocation align to the cache line size, so
+	 * certain platforms due to atomic variables from the same cache line.
+	 * This ensures that the LR/SC variables are in a separate cache line 
+	 * to avoid live lock.
+	 */
+
+	size += SCRATCH_ALLOC_ALIGNMENT- 1;
+	size &= ~((unsigned long)SCRATCH_ALLOC_ALIGNMENT - 1);
 
 	spin_lock(&extra_lock);
 
-- 
2.43.0

[PATCH v2] We add a new configuration CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT as an int

[Xiang W]

? 2025-01-19?? 12:04 -0800?Raj Vishwanathan???
> If it is set to 0 or not defined, we will continue with the previous
> defintion of allocating pointer size chunks. Otherwise we will use the
> chacheline size of 64.
> We have the option of increasing the scratch allocation alignment to 64 bytes
> as the cache line size, to avoid two atomic variables from
> the same cache line that may cause livelock on some platforms.
> ---
> ?lib/sbi/Kconfig?????? |? 8 ++++++++
> ?lib/sbi/sbi_scratch.c | 18 ++++++++++++++++--
> ?2 files changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/sbi/Kconfig b/lib/sbi/Kconfig
> index c6cc04b..1ffa170 100644
> --- a/lib/sbi/Kconfig
> +++ b/lib/sbi/Kconfig
> @@ -69,4 +69,12 @@ config SBI_ECALL_SSE
> ?config SBI_ECALL_MPXY
> ?	bool "MPXY extension"
> ?	default y
> +config SBI_SCRATCH_ALLOC_ALIGNMENT
> +	int? "Scratch allocation alignment"
> +	default 0
> +	help
> +	? We provide the option for allocation alignment to 64 bytes to avoid 
> +	? livelock on certain platforms due to atomic variables from the same 
> +	? cache line.? Leave it 0 for default behaviour.

Don't stress 64 bytes specifically

Customise the size of the alignment to allocate from the extra space in
sbi_scratch. Leave it 0 for default behaviour

Regards,
Xiang W

> +
> ?endmenu
> diff --git a/lib/sbi/sbi_scratch.c b/lib/sbi/sbi_scratch.c
> index ccbbc68..88ea3c7 100644
> --- a/lib/sbi/sbi_scratch.c
> +++ b/lib/sbi/sbi_scratch.c
> @@ -14,6 +14,13 @@
> ?#include <sbi/sbi_scratch.h>
> ?#include <sbi/sbi_string.h>
> ?
> +#if !defined(CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT) || (CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT==0)
> +#define SCRATCH_ALLOC_ALIGNMENT __SIZEOF_POINTER__
> +#else
> +#define SCRATCH_ALLOC_ALIGNMENT CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT
> +#endif
> +
> +
> ?u32 last_hartindex_having_scratch = 0;
> ?u32 hartindex_to_hartid_table[SBI_HARTMASK_MAX_BITS + 1] = { -1U };
> ?struct sbi_scratch *hartindex_to_scratch_table[SBI_HARTMASK_MAX_BITS + 1] = { 0 };
> @@ -70,8 +77,15 @@ unsigned long sbi_scratch_alloc_offset(unsigned long size)
> ?	if (!size)
> ?		return 0;
> ?
> -	size += __SIZEOF_POINTER__ - 1;
> -	size &= ~((unsigned long)__SIZEOF_POINTER__ - 1);
> +	/*
> +	 * We let the allocation align to the cache line size, so
> +	 * certain platforms due to atomic variables from the same cache line.
> +	 * This ensures that the LR/SC variables are in a separate cache line 
> +	 * to avoid live lock.
> +	 */
> +
> +	size += SCRATCH_ALLOC_ALIGNMENT- 1;
> +	size &= ~((unsigned long)SCRATCH_ALLOC_ALIGNMENT - 1);
> ?
> ?	spin_lock(&extra_lock);
> ?
> -- 
> 2.43.0
> 
> 

[EXTERNAL]Re: [PATCH v2] We add a new configuration CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT as an int

[Raj Vishwanathan]

Xiang W.
Sounds good. Will update it the text in Kconfig. The reason for 64 bytes was to align it with the?cacheline size.

Raj
-----Original Message-----
From: Xiang W <wxjstz@126.com> 
Sent: Sunday, January 19, 2025 7:58 PM
To: Raj Vishwanathan <raj.vishwanathan@gmail.com>; opensbi at lists.infradead.org
Cc: Raj Vishwanathan <rvishwanathan@mips.com>
Subject: [EXTERNAL]Re: [PATCH v2] We add a new configuration CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT as an int

[You don't often get email from wxjstz at 126.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]

? 2025-01-19?? 12:04 -0800?Raj Vishwanathan???
> If it is set to 0 or not defined, we will continue with the previous 
> defintion of allocating pointer size chunks. Otherwise we will use the 
> chacheline size of 64.
> We have the option of increasing the scratch allocation alignment to 
> 64 bytes as the cache line size, to avoid two atomic variables from 
> the same cache line that may cause livelock on some platforms.
> ---
>  lib/sbi/Kconfig       |? 8 ++++++++
>  lib/sbi/sbi_scratch.c | 18 ++++++++++++++++--
>  2 files changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/lib/sbi/Kconfig b/lib/sbi/Kconfig index c6cc04b..1ffa170 
> 100644
> --- a/lib/sbi/Kconfig
> +++ b/lib/sbi/Kconfig
> @@ -69,4 +69,12 @@ config SBI_ECALL_SSE  config SBI_ECALL_MPXY
>       bool "MPXY extension"
>       default y
> +config SBI_SCRATCH_ALLOC_ALIGNMENT
> +     int  "Scratch allocation alignment"
> +     default 0
> +     help
> +       We provide the option for allocation alignment to 64 bytes to avoid
> +       livelock on certain platforms due to atomic variables from the same
> +       cache line.  Leave it 0 for default behaviour.

Don't stress 64 bytes specifically

Customise the size of the alignment to allocate from the extra space in sbi_scratch. Leave it 0 for default behaviour

Regards,
Xiang W

> +
>  endmenu
> diff --git a/lib/sbi/sbi_scratch.c b/lib/sbi/sbi_scratch.c index 
> ccbbc68..88ea3c7 100644
> --- a/lib/sbi/sbi_scratch.c
> +++ b/lib/sbi/sbi_scratch.c
> @@ -14,6 +14,13 @@
>  #include <sbi/sbi_scratch.h>
>  #include <sbi/sbi_string.h>
>
> +#if !defined(CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT) || 
> +(CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT==0)
> +#define SCRATCH_ALLOC_ALIGNMENT __SIZEOF_POINTER__ #else #define 
> +SCRATCH_ALLOC_ALIGNMENT CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT
> +#endif
> +
> +
>  u32 last_hartindex_having_scratch = 0;
>  u32 hartindex_to_hartid_table[SBI_HARTMASK_MAX_BITS + 1] = { -1U };  
> struct sbi_scratch *hartindex_to_scratch_table[SBI_HARTMASK_MAX_BITS + 
> 1] = { 0 }; @@ -70,8 +77,15 @@ unsigned long sbi_scratch_alloc_offset(unsigned long size)
>       if (!size)
>               return 0;
>
> -     size += __SIZEOF_POINTER__ - 1;
> -     size &= ~((unsigned long)__SIZEOF_POINTER__ - 1);
> +     /*
> +      * We let the allocation align to the cache line size, so
> +      * certain platforms due to atomic variables from the same cache line.
> +      * This ensures that the LR/SC variables are in a separate cache line
> +      * to avoid live lock.
> +      */
> +
> +     size += SCRATCH_ALLOC_ALIGNMENT- 1;
> +     size &= ~((unsigned long)SCRATCH_ALLOC_ALIGNMENT - 1);
>
>       spin_lock(&extra_lock);
>
> --
> 2.43.0
>
>

[PATCH v3] New configuration CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT

[Raj Vishwanathan]

We add a new configuration CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT as an int
If it is set to 0 or not defined, we will continue with the previous
defintion of allocating pointer size chunks. Otherwise we will use the
chacheline size of 64.
We have the option of increasing the scratch allocation alignment to 64 bytes
as the cache line size, to avoid two atomic variables from
the same cache line that may cause livelock on some platforms.

Update: Agreeing with the reviewer's comment about not stressing 64 bytes.
---
 lib/sbi/Kconfig       |  7 +++++++
 lib/sbi/sbi_scratch.c | 18 ++++++++++++++++--
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/lib/sbi/Kconfig b/lib/sbi/Kconfig
index c6cc04b..5f7eb70 100644
--- a/lib/sbi/Kconfig
+++ b/lib/sbi/Kconfig
@@ -69,4 +69,11 @@ config SBI_ECALL_SSE
 config SBI_ECALL_MPXY
 	bool "MPXY extension"
 	default y
+config SBI_SCRATCH_ALLOC_ALIGNMENT
+	int  "Scratch allocation alignment"
+	default 0
+	help
+	  We provide the option to customize the alignment to allocate from
+	  the extra space in sbi_scratch. Leave it 0 for default behaviour.
+
 endmenu
diff --git a/lib/sbi/sbi_scratch.c b/lib/sbi/sbi_scratch.c
index ccbbc68..88ea3c7 100644
--- a/lib/sbi/sbi_scratch.c
+++ b/lib/sbi/sbi_scratch.c
@@ -14,6 +14,13 @@
 #include <sbi/sbi_scratch.h>
 #include <sbi/sbi_string.h>
 
+#if !defined(CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT) || (CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT==0)
+#define SCRATCH_ALLOC_ALIGNMENT __SIZEOF_POINTER__
+#else
+#define SCRATCH_ALLOC_ALIGNMENT CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT
+#endif
+
+
 u32 last_hartindex_having_scratch = 0;
 u32 hartindex_to_hartid_table[SBI_HARTMASK_MAX_BITS + 1] = { -1U };
 struct sbi_scratch *hartindex_to_scratch_table[SBI_HARTMASK_MAX_BITS + 1] = { 0 };
@@ -70,8 +77,15 @@ unsigned long sbi_scratch_alloc_offset(unsigned long size)
 	if (!size)
 		return 0;
 
-	size += __SIZEOF_POINTER__ - 1;
-	size &= ~((unsigned long)__SIZEOF_POINTER__ - 1);
+	/*
+	 * We let the allocation align to the cache line size, so
+	 * certain platforms due to atomic variables from the same cache line.
+	 * This ensures that the LR/SC variables are in a separate cache line 
+	 * to avoid live lock.
+	 */
+
+	size += SCRATCH_ALLOC_ALIGNMENT- 1;
+	size &= ~((unsigned long)SCRATCH_ALLOC_ALIGNMENT - 1);
 
 	spin_lock(&extra_lock);
 
-- 
2.43.0

[PATCH v3] New configuration CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT

[Xiang W]

? 2025-01-20?? 17:46 -0800?Raj Vishwanathan???
> We add a new configuration CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT as an int
> If it is set to 0 or not defined, we will continue with the previous
> defintion of allocating pointer size chunks. Otherwise we will use the
> chacheline size of 64.
> We have the option of increasing the scratch allocation alignment to 64 bytes
> as the cache line size, to avoid two atomic variables from
> the same cache line that may cause livelock on some platforms.
> 
missing your Signed-off-by
> Update: Agreeing with the reviewer's comment about not stressing 64 bytes.
The previous line need move after '---' 

> ---
> ?lib/sbi/Kconfig?????? |? 7 +++++++
> ?lib/sbi/sbi_scratch.c | 18 ++++++++++++++++--
> ?2 files changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/sbi/Kconfig b/lib/sbi/Kconfig
> index c6cc04b..5f7eb70 100644
> --- a/lib/sbi/Kconfig
> +++ b/lib/sbi/Kconfig
> @@ -69,4 +69,11 @@ config SBI_ECALL_SSE
> ?config SBI_ECALL_MPXY
> ?	bool "MPXY extension"
> ?	default y
> +config SBI_SCRATCH_ALLOC_ALIGNMENT
> +	int? "Scratch allocation alignment"
> +	default 0
> +	help
> +	? We provide the option to customize the alignment to allocate from
> +	? the extra space in sbi_scratch. Leave it 0 for default behaviour.
> +
> ?endmenu
> diff --git a/lib/sbi/sbi_scratch.c b/lib/sbi/sbi_scratch.c
> index ccbbc68..88ea3c7 100644
> --- a/lib/sbi/sbi_scratch.c
> +++ b/lib/sbi/sbi_scratch.c
> @@ -14,6 +14,13 @@
> ?#include <sbi/sbi_scratch.h>
> ?#include <sbi/sbi_string.h>
> ?
> +#if !defined(CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT) || (CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT==0)
> +#define SCRATCH_ALLOC_ALIGNMENT __SIZEOF_POINTER__
> +#else
> +#define SCRATCH_ALLOC_ALIGNMENT CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT
> +#endif
> +
> +
> ?u32 last_hartindex_having_scratch = 0;
> ?u32 hartindex_to_hartid_table[SBI_HARTMASK_MAX_BITS + 1] = { -1U };
> ?struct sbi_scratch *hartindex_to_scratch_table[SBI_HARTMASK_MAX_BITS + 1] = { 0 };
> @@ -70,8 +77,15 @@ unsigned long sbi_scratch_alloc_offset(unsigned long size)
> ?	if (!size)
> ?		return 0;
> ?
> -	size += __SIZEOF_POINTER__ - 1;
> -	size &= ~((unsigned long)__SIZEOF_POINTER__ - 1);
> +	/*
> +	 * We let the allocation align to the cache line size, so
> +	 * certain platforms due to atomic variables from the same cache line.
> +	 * This ensures that the LR/SC variables are in a separate cache line 
> +	 * to avoid live lock.
> +	 */
These comments are platform-specific. Better change it.

Regards,
Xiang W
> +
> +	size += SCRATCH_ALLOC_ALIGNMENT- 1;
> +	size &= ~((unsigned long)SCRATCH_ALLOC_ALIGNMENT - 1);
> ?
> ?	spin_lock(&extra_lock);
> ?
> -- 
> 2.43.0
> 
> 

[PATCH v3] New configuration CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT

[Raj Vishwanathan]

We add a new configuration CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT as an int
If it is set to 0 or not defined, we will continue with the previous
defintion of allocating pointer size chunks. Otherwise we will use the
chacheline size of 64.
We have the option of increasing the scratch allocation alignment to 64 bytes
as the cache line size, to avoid two atomic variables from
the same cache line that may cause livelock on some platforms.

---
Update: Agreeing with the reviewer's comment about not stressing 64 bytes.

Signed-off-by: Raj Vishwanathan <Raj.Vishwanathan@gmail.com>
---
 lib/sbi/Kconfig       |  7 +++++++
 lib/sbi/sbi_scratch.c | 18 ++++++++++++++++--
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/lib/sbi/Kconfig b/lib/sbi/Kconfig
index c6cc04b..5f7eb70 100644
--- a/lib/sbi/Kconfig
+++ b/lib/sbi/Kconfig
@@ -69,4 +69,11 @@ config SBI_ECALL_SSE
 config SBI_ECALL_MPXY
 	bool "MPXY extension"
 	default y
+config SBI_SCRATCH_ALLOC_ALIGNMENT
+	int  "Scratch allocation alignment"
+	default 0
+	help
+	  We provide the option to customize the alignment to allocate from
+	  the extra space in sbi_scratch. Leave it 0 for default behaviour.
+
 endmenu
diff --git a/lib/sbi/sbi_scratch.c b/lib/sbi/sbi_scratch.c
index ccbbc68..3fd7b4b 100644
--- a/lib/sbi/sbi_scratch.c
+++ b/lib/sbi/sbi_scratch.c
@@ -14,6 +14,13 @@
 #include <sbi/sbi_scratch.h>
 #include <sbi/sbi_string.h>
 
+#if !defined(CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT) || (CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT==0)
+#define SCRATCH_ALLOC_ALIGNMENT __SIZEOF_POINTER__
+#else
+#define SCRATCH_ALLOC_ALIGNMENT CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT
+#endif
+
+
 u32 last_hartindex_having_scratch = 0;
 u32 hartindex_to_hartid_table[SBI_HARTMASK_MAX_BITS + 1] = { -1U };
 struct sbi_scratch *hartindex_to_scratch_table[SBI_HARTMASK_MAX_BITS + 1] = { 0 };
@@ -70,8 +77,15 @@ unsigned long sbi_scratch_alloc_offset(unsigned long size)
 	if (!size)
 		return 0;
 
-	size += __SIZEOF_POINTER__ - 1;
-	size &= ~((unsigned long)__SIZEOF_POINTER__ - 1);
+	/*
+	 * We do the scratch allocation on the basis of a user configuration.
+	 * We have the option of increasing the scratch allocation alignment
+	 * to the cache line size (64 bytes) to avoid two atomic variables
+	 * from the same cache line that may cause livelock on some platforms.
+	 */
+
+	size += SCRATCH_ALLOC_ALIGNMENT- 1;
+	size &= ~((unsigned long)SCRATCH_ALLOC_ALIGNMENT - 1);
 
 	spin_lock(&extra_lock);
 
-- 
2.43.0

[PATCH v3] New configuration CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT

[Raj Vishwanathan]

We add a new configuration CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT as an int
If it is set to 0 or not defined, we will continue with the previous
defintion of allocating pointer size chunks. Otherwise we will use the
user specified size.
We use the scratch allocation alignment to 64 bytes or cacheline size,
to avoid two atomic variables from the same cache line causing livelock
on some platforms.

Changes in V2:
	Remove platform specific references to 64 bytes in the configuration

Changes in V3:
	Remove platform specific references in 64 bytes in the comments

Signed-off-by: Raj Vishwanathan <Raj.Vishwanathan@gmail.com>
---
 lib/sbi/Kconfig       |  7 +++++++
 lib/sbi/sbi_scratch.c | 14 ++++++++++++--
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/lib/sbi/Kconfig b/lib/sbi/Kconfig
index c6cc04b..5f7eb70 100644
--- a/lib/sbi/Kconfig
+++ b/lib/sbi/Kconfig
@@ -69,4 +69,11 @@ config SBI_ECALL_SSE
 config SBI_ECALL_MPXY
 	bool "MPXY extension"
 	default y
+config SBI_SCRATCH_ALLOC_ALIGNMENT
+	int  "Scratch allocation alignment"
+	default 0
+	help
+	  We provide the option to customize the alignment to allocate from
+	  the extra space in sbi_scratch. Leave it 0 for default behaviour.
+
 endmenu
diff --git a/lib/sbi/sbi_scratch.c b/lib/sbi/sbi_scratch.c
index ccbbc68..0f1be58 100644
--- a/lib/sbi/sbi_scratch.c
+++ b/lib/sbi/sbi_scratch.c
@@ -14,6 +14,16 @@
 #include <sbi/sbi_scratch.h>
 #include <sbi/sbi_string.h>
 
+/*
+ * We do the scratch allocation on the basis of a user configuration.
+ */
+#if !defined(CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT) || (CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT==0)
+#define SCRATCH_ALLOC_ALIGNMENT __SIZEOF_POINTER__
+#else
+#define SCRATCH_ALLOC_ALIGNMENT CONFIG_SBI_SCRATCH_ALLOC_ALIGNMENT
+#endif
+
+
 u32 last_hartindex_having_scratch = 0;
 u32 hartindex_to_hartid_table[SBI_HARTMASK_MAX_BITS + 1] = { -1U };
 struct sbi_scratch *hartindex_to_scratch_table[SBI_HARTMASK_MAX_BITS + 1] = { 0 };
@@ -70,8 +80,8 @@ unsigned long sbi_scratch_alloc_offset(unsigned long size)
 	if (!size)
 		return 0;
 
-	size += __SIZEOF_POINTER__ - 1;
-	size &= ~((unsigned long)__SIZEOF_POINTER__ - 1);
+	size += SCRATCH_ALLOC_ALIGNMENT- 1;
+	size &= ~((unsigned long)SCRATCH_ALLOC_ALIGNMENT - 1);
 
 	spin_lock(&extra_lock);
 
-- 
2.43.0