Re: [edk2-devel] [PATCH 1/5] MdePkg/BasePrintLib: avoid absolute addresses for error strings

["Ard Biesheuvel" <ard.biesheuvel@arm.com>]

On 6/10/20 5:09 PM, Michael D Kinney via groups.io wrote:
> Hi Ard,
> 
> The size reduction is very interesting.  Would be good to
> see uncompressed size differences as well because this lib
> is also used in pre-memory XIP modules that are fixed up
> by the build tools that handle the relocation fixups.
> 

As I said in my reply-to-self, this results in a net decrease in both cases:

 >> FV Space Information
 >>
 >> FVMAIN [99%Full] 4784768 total, 4784720 used, 48 free
 >> FVMAIN_COMPACT [37%Full] 2093056 total, 792984 used,
 >> 1300072 free
 >>
 >> to
 >>
 >> FV Space Information
 >> FVMAIN [99%Full] 4780672 total, 4780624 used, 48 free
 >> FVMAIN_COMPACT [37%Full] 2093056 total, 791072 used,
 >> 1301984 free
 >>

FVMAIN is the uncompressed primary FV

FVMAIN_COMPACT contains FVMAIN in compressed form, along with the PEI 
and SEC modules.

So the XIP case is a win as well.

> The more general concern I have is if there are standard
> C coding practices (such as a pre-initialized array of
> pointers to strings) that are not compatible with
> StandaloneMmPkg use cases.
> 

In general, yes. In this case, it is only about being able to make it to 
the PE/COFF relocation routines and back, which happens very early (in a 
AARCH64 specific StMM core entrypoint library, mind you). So using 
statically initialized pointers is fine in general, just not in the 
early startup code that runs before relocation.




>> -----Original Message-----
>> From: devel@edk2.groups.io <devel@edk2.groups.io> On
>> Behalf Of Ard Biesheuvel
>> Sent: Wednesday, June 10, 2020 1:37 AM
>> To: devel@edk2.groups.io
>> Cc: Kinney, Michael D <michael.d.kinney@intel.com>; Gao,
>> Liming <liming.gao@intel.com>; Yao, Jiewen
>> <jiewen.yao@intel.com>; Sami Mujawar
>> <sami.mujawar@arm.com>; Ilias Apalodimas
>> <ilias.apalodimas@linaro.org>
>> Subject: Re: [edk2-devel] [PATCH 1/5]
>> MdePkg/BasePrintLib: avoid absolute addresses for error
>> strings
>>
>> On 6/10/20 10:17 AM, Ard Biesheuvel wrote:
>>> The mStatusString[] is constructed as an array of
>> pointer-to-char, which
>>> means that on X64 or AARCH64, it is emitted as a
>> single linear list of
>>> 64-bit quantities, each containing the absolute
>> address of one of the
>>> string literals in memory.
>>>
>>> This means that each string takes up 8 bytes of
>> additional space, along
>>> with 2 bytes of relocation data. It also means that
>> the array cannot be
>>> used until PE/COFF relocation has completed, and so
>> the following
>>> invocation
>>>
>>>       Status = PeCoffLoaderRelocateImage
>> (&ImageContext);
>>>       ASSERT_EFI_ERROR (Status);
>>>
>>> that we will be introducing into StandaloneMmCore
>> entrypoint for AARCH64
>>> to relocate the executable on the fly is guaranteed to
>> return bogus output
>>> or crash, which is less than helpful.
>>>
>>> So fix both issues, by emitting mStatusString as an
>> array of char arrays
>>> instead. The memory footprint increases from 955 to
>> 975 bytes, but given
>>> that in the latter case, the overhead consists of 410
>> NUL characters
>>> rather than 390 bytes worth of absolute addresses and
>> relocation records,
>>> the impact on a compressed image is actually positive.
>> For example, when
>>> building ArmVirtQemu.dsc in RELEASE mode for AARCH64
>> with the GCC5 profile,
>>> I get:
>>>
>>>     Before
>>>
>>>     FV Space Information
>>>     FVMAIN [99%Full] 4784768 total, 4784720 used, 48
>> free
>>>     FVMAIN_COMPACT [38%Full] 2093056 total, 811560
>> used, 1281496 free
>>>
>>>     After
>>>
>>>     FV Space Information
>>>     FVMAIN [99%Full] 4780672 total, 4780624 used, 48
>> free
>>>     FVMAIN_COMPACT [38%Full] 2093056 total, 813488
>> used, 1279568 free
>>>
>>> So the compressed image is 4 KB smaller, whereas the
>> entire image is
>>> < 2 KB larger, which is in the order of 0.2 %
>>>
>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
>>
>> Actually, I found a slightly better way of doing this,
>> by applying the
>> following delta on top, i.e., split the errors and
>> warnings, and use an
>> array of 21 byte character arrays for the former.
>>
>> My build went from
>>
>> FV Space Information
>>
>> FVMAIN [99%Full] 4784768 total, 4784720 used, 48 free
>> FVMAIN_COMPACT [37%Full] 2093056 total, 792984 used,
>> 1300072 free
>>
>> to
>>
>> FV Space Information
>> FVMAIN [99%Full] 4780672 total, 4780624 used, 48 free
>> FVMAIN_COMPACT [37%Full] 2093056 total, 791072 used,
>> 1301984 free
>>
>>
>> in this case, i.e., both compressed and non-compressed
>> images are
>> slightly smaller.
>>
>>
>>
>> --- a/MdePkg/Library/BasePrintLib/PrintLibInternal.c
>> +++ b/MdePkg/Library/BasePrintLib/PrintLibInternal.c
>> @@ -27,13 +27,16 @@
>>
>>    GLOBAL_REMOVE_IF_UNREFERENCED CONST CHAR8 mHexStr[] =
>> {'0','1','2','3','4','5','6','7','8','9','A','B','C','D'
>> ,'E','F'};
>>
>> -GLOBAL_REMOVE_IF_UNREFERENCED CONST CHAR8
>> mStatusString[][25] = {
>> +GLOBAL_REMOVE_IF_UNREFERENCED CONST CHAR8
>> mWarningString[][25] = {
>>      "Success",                      //  RETURN_SUCCESS
>> = 0
>>      "Warning Unknown Glyph",        //
>> RETURN_WARN_UNKNOWN_GLYPH     = 1
>>      "Warning Delete Failure",       //
>> RETURN_WARN_DELETE_FAILURE    = 2
>>      "Warning Write Failure",        //
>> RETURN_WARN_WRITE_FAILURE     = 3
>>      "Warning Buffer Too Small",     //
>> RETURN_WARN_BUFFER_TOO_SMALL  = 4
>>      "Warning Stale Data",           //
>> RETURN_WARN_STALE_DATA        = 5
>> +};
>> +
>> +GLOBAL_REMOVE_IF_UNREFERENCED CONST CHAR8
>> mErrorString[][21] = {
>>      "Load Error",                   //
>> RETURN_LOAD_ERROR             =
>> 1  | MAX_BIT
>>      "Invalid Parameter",            //
>> RETURN_INVALID_PARAMETER      =
>> 2  | MAX_BIT
>>      "Unsupported",                  //
>> RETURN_UNSUPPORTED            =
>> 3  | MAX_BIT
>> @@ -996,12 +999,12 @@ BasePrintLibSPrintMarker (
>>              //
>>              Index = Status & ~MAX_BIT;
>>              if (Index > 0 && Index <=
>> ERROR_STATUS_NUMBER) {
>> -            ArgumentString = mStatusString [Index +
>> WARNING_STATUS_NUMBER];
>> +            ArgumentString = mErrorString [Index - 1];
>>              }
>>            } else {
>>              Index = Status;
>>              if (Index <= WARNING_STATUS_NUMBER) {
>> -            ArgumentString = mStatusString [Index];
>> +            ArgumentString = mWarningString [Index];
>>              }
>>            }
>>            if (ArgumentString == ValueBuffer) {
>>
>>
>>> ---
>>>    MdePkg/Library/BasePrintLib/PrintLibInternal.c | 2
>> +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git
>> a/MdePkg/Library/BasePrintLib/PrintLibInternal.c
>> b/MdePkg/Library/BasePrintLib/PrintLibInternal.c
>>> index b6ec5ac4fbb9..c8b932c7e07a 100644
>>> --- a/MdePkg/Library/BasePrintLib/PrintLibInternal.c
>>> +++ b/MdePkg/Library/BasePrintLib/PrintLibInternal.c
>>> @@ -27,7 +27,7 @@
>>>
>>>    GLOBAL_REMOVE_IF_UNREFERENCED CONST CHAR8 mHexStr[]
>> =
>> {'0','1','2','3','4','5','6','7','8','9','A','B','C','D'
>> ,'E','F'};
>>>
>>> -GLOBAL_REMOVE_IF_UNREFERENCED CONST CHAR8 * CONST
>> mStatusString[] = {
>>> +GLOBAL_REMOVE_IF_UNREFERENCED CONST CHAR8
>> mStatusString[][25] = {
>>>      "Success",                      //  RETURN_SUCCESS
>> = 0
>>>      "Warning Unknown Glyph",        //
>> RETURN_WARN_UNKNOWN_GLYPH     = 1
>>>      "Warning Delete Failure",       //
>> RETURN_WARN_DELETE_FAILURE    = 2
>>>
>>
>>
>>
> 
> 
> 
>