Re: [patch] MdeModulePkg/DriverSampleDxe: Make bit fields aligned in C structure

["Gao, Liming" <liming.gao@intel.com>]

Reviewed-by: Liming Gao <liming.gao@intel.com>

>-----Original Message-----
>From: Bi, Dandan
>Sent: Thursday, February 08, 2018 10:29 PM
>To: edk2-devel@lists.01.org
>Cc: Dong, Eric <eric.dong@intel.com>; Gao, Liming <liming.gao@intel.com>
>Subject: [patch] MdeModulePkg/DriverSampleDxe: Make bit fields aligned in
>C structure
>
>For a structure with a series of bit fields and used as a storage
>in vfr file, and if the bit fields do not add up to the size of
>the defined type.In the C code use sizeof() to get size of the
>structure, the results may vary form the compiler(VS,GCC...).
>But the size of the storage calculated by VfrCompiler is fixed
>(calculate with alignment).To avoid the issue cased by above case,
>we need to make the total width of the bit fields in the structure
>aligned with the size of the defined type for these bit fields.
>
>Cc: Eric Dong <eric.dong@intel.com>
>Cc: Liming Gao <liming.gao@intel.com>
>Contributed-under: TianoCore Contribution Agreement 1.1
>Signed-off-by: Dandan Bi <dandan.bi@intel.com>
>---
> MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h | 11
>+++++++++++
> 1 file changed, 11 insertions(+)
>
>diff --git a/MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h
>b/MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h
>index 40fb3d0..af3d4bc 100644
>--- a/MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h
>+++ b/MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h
>@@ -33,10 +33,18 @@ Revision History:
> #define CONFIGURATION_VARSTORE_ID    0x1234
> #define BITS_VARSTORE_ID             0x2345
>
> #pragma pack(1)
>
>+//
>+// !!! For a structure with a series of bit fields and used as a storage in vfr file,
>and if the bit fields do not add up to the size of the defined type.
>+// In the C code use sizeof() to get the size the strucure, the results may vary
>form the compiler(VS,GCC...).
>+// But the size of the storage calculated by VfrCompiler is fixed (calculate
>with alignment).
>+// To avoid above case, we need to make the total bit width in the structure
>aligned with the size of the defined type for these bit fields. We can:
>+// 1. Add bit field (with/without name) with remianing with for padding.
>+// 2. Add unnamed bit field with 0 for padding, the amount of padding is
>determined by the alignment characteristics of the members of the structure.
>+//
> typedef struct {
>   UINT16   NestByteField;
>   UINT8                    : 1;  // unamed field can be used for padding
>   UINT8    NestBitCheckbox : 1;
>   UINT8    NestBitOneof    : 2;
>@@ -82,11 +90,13 @@ typedef struct {
>   EFI_HII_TIME  Time;
>   UINT8   RefreshGuidCount;
>   UINT8   Match2;
>   UINT8   GetDefaultValueFromCallBackForOrderedList[3];
>   UINT8   BitCheckbox : 1;
>+  UINT8   ReservedBits: 7;  // Reserved bit fields for padding.
>   UINT16  BitOneof    : 6;
>+  UINT16              : 0;  // Width 0 used to force alignment.
>   UINT16  BitNumeric  : 12;
>   MY_BITS_DATA  MyBitData;
>   MY_EFI_UNION_DATA MyUnionData;
> } DRIVER_SAMPLE_CONFIGURATION;
>
>@@ -107,10 +117,11 @@ typedef struct {
>   MY_BITS_DATA  BitsData;
>   UINT32   EfiBitGrayoutTest : 5;
>   UINT32   EfiBitNumeric     : 4;
>   UINT32   EfiBitOneof       : 10;
>   UINT32   EfiBitCheckbox    : 1;
>+  UINT32                     : 0;  // Width 0 used to force alignment.
> } MY_EFI_BITS_VARSTORE_DATA;
>
> //
> // Labels definition
> //
>--
>1.9.5.msysgit.1