This is the mail archive of the mailing list for the glibc project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

[RFC] dl-procinfo and HWCAP_IMPORTANT support for powerpc, revised

This RFC has been through the wash and spin cycles a couple of times with 
the wider community and I think is in final form. If ther are no 
objections I will submit a small patch for libc/powerpc to match the 
proposal below.


The recently committed dl-procinfo support for powerpc provides names for 
AT_HWCAP bits as of 2.6.15. This update also defines HWCAP_IMPORTANT 
support that allows for HWCAP based extensions to the library search path. 
The intent is to allow the loader ( to select the most appropriate 
version of a library given the hardware we are running on. This is 
equivalent the support for i686 optimized libraries for the i386 platform.

The 2.6.15 kernel defines the following AT_HWCAP bits:

#define PPC_FEATURE_32             0x80000000 /* 32-bit mode. */
#define PPC_FEATURE_64             0x40000000 /* 64-bit mode. */
#define PPC_FEATURE_601_INSTR      0x20000000 /* 601 chip, Old POWER ISA. 
#define PPC_FEATURE_HAS_ALTIVEC    0x10000000 /* SIMD/Vector Unit.  */
#define PPC_FEATURE_HAS_FPU        0x08000000 /* Floating Point Unit.  */
#define PPC_FEATURE_HAS_MMU        0x04000000 /* Memory Management Unit. 
#define PPC_FEATURE_HAS_4xxMAC     0x02000000 /* 4xx Multiply Accumulator. 
#define PPC_FEATURE_UNIFIED_CACHE  0x01000000 /* Unified I/D cache.  */
#define PPC_FEATURE_HAS_SPE        0x00800000 /* Signal Processing 
Extentions.  */
#define PPC_FEATURE_HAS_EFP_SINGLE 0x00400000 /* SPE Float.  */
#define PPC_FEATURE_HAS_EFP_DOUBLE 0x00200000 /* SPE Double.  */
#define PPC_FEATURE_NO_TB               0x00100000 /* 601/403gx have no 
timebase.  */
#define PPC_FEATURE_POWER4              0x00080000 /* POWER4 ISA 2.00 
level.  */
#define PPC_FEATURE_POWER5              0x00040000 /* POWER5 ISA 2.01 
level.  */
#define PPC_FEATURE_POWER5_PLUS 0x00020000 /* POWER5+ ISA 2.02 level.  */
#define PPC_FEATURE_CELL_BE             0x00010000 /* CELL Broadband 
Engine.  */

To which we will add the following:
#define PPC_FEATURE_BOOKE          0x00008000 /* BOOKE ISA features.  */

These bits where given the following procinfo names

To which we will change/add the following: "cell" will become "cellbe", 
add "booke".

The initial proposal was sufficient for current needs of PowerPC 64-bit, 
but concern was expressed that PowerPC 32-bit and embedded where not 
covered. Also the concern was raised that HWCAP was a finite resource that 
might be exhausted by the large variety of 32-bit PowerPC (cpu_type) 
variants and future 64-bit processors.

It was suggested that AT_PLATFORM is a better mechanism for supporting a 
large variety of cpu-types for compiler (micro-architecture) 
optimizations. And AT_HWCAP should be focused on ISA features and optional 
execution units.

still useful because they represent ISA Version levels where each level 
introduces new user state instructions:

   PPC_FEATURE_POWER4      -> PowerPC ISA Version 2.00
   PPC_FEATURE_POWER5      -> PowerPC ISA Version 2.01
   PPC_FEATURE_POWER5_PLUS -> PowerPC ISA Version 2.02

Perhaps these should be renamed or aliased, to something like 
PPC_FEATURE_ISA_POWER4, but this is not critical to this (dl-procinfo) 
proposal. For library search these are redundant with the AT_PLATFORM 
information, so they will be removed from the HWCAP_IMPORTANT mask.

But for the embedded space there we need a HWCAP bit to identify the user 
mode "isel", "mbar", "msync", etc ISA feature associated with BOOKE. So we 
propose adding HWCAP bit for PPC_FEATURE_BOOKE to represent this ISA 
extention common in the embedded space. 

The new HWCAP_IMPORTANT mask is:

+                       | PPC_FEATURE_CELL_BE       \
+                       | PPC_FEATURE_BOOKE)

With this change AT_PLATFORM provides the cpu_type selection while 
AT_HWCAP provides ISA modifiers for the ALTIVEC, CELL_BE, and BOOKE 
features. ALTIVEC, CELL_BE, BOOKE are exclusive features so a maximum 2 
directory levels are introduced.

The specific strings provided in AT_PLATFORM are "inspired" by the gcc 
-mcpu= cpu_type names.  Self defining names are used as is (power4, 
power5, ...). For AT_PLATFORM numeric chip_type names (970, 7450, ...) are 
prefixed with "ppc" (ppc970, ppc7450). Where the cpu_type has aliases (G5 
is an alias for 970) the more generic name is used (i.e. ppc970 not G5 and 
ppc7450 not G4).

For 64-bit processors and kernels we can assume (at least) the following 
AT_PLATFORM names:  "rs64", "power3", "power4", "power5", "power5+", 
"ppc970" (AKA G5), and "ppc-cell-be". 

For 32-bit processors and kernels we propose a longer list as dictated by 
the cpu_types the kernel supports. The current list is: "ppc403", 
"ppc405", "ppc440", "ppc440GP", "ppc601", "ppc603", "ppc604", "ppc7400", 
"ppc7450" (AKA G4), "ppc750" (AKA G3), "ppc823", "ppc8540".

Note: the "powerpc" value is reserved and only used for the case where the 
PVR value is unrecognized.

The dl-procinfo implementation uses this information to augment the 
library search list. The proposed correspondence between processors and 
runtime library search directories (assuming the current nptl paths) are:

processor     library search
==========    ============
power4        /lib[64]/tls/power4, /lib[64]/tls, /lib[64]
power5        /lib[64]/tls/power5, /lib[64]/tls, /lib[64]
power5+       /lib[64]/tls/power5+, /lib[64]/tls, /lib[64]
970/G5        /lib[64]/tls/ppc970/altivec, /lib[64]/tls/ppc970, 
              /lib[64]/tls, /lib[64]

7450/G4       /lib/tls/ppc7450/altivec, /lib/tls/ppc7450, 
/lib/tls/altivec, /lib/tls, /lib
750/G3        /lib/tls/ppc750, /lib/tls, /lib
440           /lib/tls/ppc440/booke, /lib/tls/ppc440, /lib/tls/booke,
              /lib/tls, /lib

Note: The additional "altivec" level for the 970/7450 is an artifact of 
the PPC_FEATURE_HAS_ALTIVEC modifier from AT_HWCAP. The "altivec" 
directory can be used to store Altivec optimized libraries that are common 
across platforms. The simularly for the "booke" level for the 440 is an 
artifact of PPC_FEATURE_BOOKE.

Steven J. Munroe
Linux on Power Toolchain Architect
IBM Corporation, Linux Technology Center

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]