Re: [PATCH v5 2/6] iommu/amd: Introduce helper function to update 256-bit DTE

[Uros Bizjak]

On 7. 10. 24 06:13, Suravee Suthikulpanit wrote:

> +
>   /****************************************************************************
>    *
>    * Helper functions
>    *
>    ****************************************************************************/
>   
> +static void write_dte_upper128(struct dev_table_entry *ptr, struct dev_table_entry *new)
> +{
> +	struct dev_table_entry old = {};
> +
> +	do {
> +		old.data128[1] = ptr->data128[1];
> +		new->data[2] &= ~DTE_DATA2_INTR_MASK;
> +		new->data[2] |= old.data[2] & (DTE_DATA2_INTR_MASK | DTE_DATA2_RESV_MASK);
> +	} while (!try_cmpxchg128(&ptr->data128[1], &old.data128[1], new->data128[1]));

Please note that try_cmpxchg inherently updates &old.data128[1] above on 
failure. There is no need to update value again in the loop.

Please also note that the value from ptr->data128[1] should be read 
using READ_ONCE() to prevent compiler from merging, refetching or 
reordering the read. Currently, there is no READ_ONCE() implemented for 
__int128, so something like the attached patch should be used.

Based on the above, the loop should be rewritten as:

	old.data128[1] = READ_ONCE(ptr->data128[1]);
	do {
		new->data[2] &= ~DTE_DATA2_INTR_MASK;
		new->data[2] |= old.data[2] & (DTE_DATA2_INTR_MASK | DTE_DATA2_RESV_MASK);
	} while (!try_cmpxchg128(&ptr->data128[1], &old.data128[1], 
new->data128[1]));

> +}
> +
> +static void write_dte_lower128(struct dev_table_entry *ptr, struct dev_table_entry *new)
> +{
> +	struct dev_table_entry old = {};
> +
> +	/*
> +	 * Need to preserve DTE[96:106], which can be set by information in IVRS table.
> +	 * See set_dev_entry_from_acpi().
> +	 */
> +	new->data[1] |= ptr->data[1] & DTE_FLAG_MASK;
> +
> +	do {
> +		old.data128[0] = ptr->data128[0];
> +	} while (!try_cmpxchg128(&ptr->data128[0], &old.data128[0], new->data128[0]));

And this one as:

	old.data128[0] = READ_ONCE(ptr->data128[0]);
	do {
	} while (!try_cmpxchg128(&ptr->data128[0], &old.data128[0], 
new->data128[0]));

Best regards,
Uros.diff --git a/include/asm-generic/rwonce.h b/include/asm-generic/rwonce.h
index 8d0a6280e982..8bf942ad5ef3 100644
--- a/include/asm-generic/rwonce.h
+++ b/include/asm-generic/rwonce.h
@@ -33,7 +33,7 @@
  * (e.g. a virtual address) and a strong prevailing wind.
  */
 #define compiletime_assert_rwonce_type(t)					\
-	compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long),	\
+	compiletime_assert(__native_word(t) || sizeof(t) == sizeof(__dword_type), \
 		"Unsupported access size for {READ,WRITE}_ONCE().")
 
 /*
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 94b8fedfb077..8615e91f48fd 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -469,6 +469,12 @@ struct ftrace_likely_data {
 		unsigned type:	(unsigned type)0,			\
 		signed type:	(signed type)0
 
+#ifdef __SIZEOF_INT128__
+#define __dword_type __int128
+#else
+#define __dword_type long long
+#endif
+
 #define __unqual_scalar_typeof(x) typeof(				\
 		_Generic((x),						\
 			 char:	(char)0,				\
@@ -476,7 +482,7 @@ struct ftrace_likely_data {
 			 __scalar_type_to_expr_cases(short),		\
 			 __scalar_type_to_expr_cases(int),		\
 			 __scalar_type_to_expr_cases(long),		\
-			 __scalar_type_to_expr_cases(long long),	\
+			 __scalar_type_to_expr_cases(__dword_type),	\
 			 default: (x)))
 
 /* Is this type a native word size -- useful for atomic operations */