The lack of emulation support could constrain developers targeting Red Hat Enterprise Linux 9.Īs in previous Red Hat Enterprise Linux versions, we will continue to support other CPU features (beyond x86-64-v2) via IFUNCs and function multi-versioning.
REDHAT LINUX 9 SOFTWARE
AVX instructions are also unavailable in certain software implementations (although the valgrind tool supports them).The new server-class CPUs released in 2020 do not implement the AVX instruction set.The next level, x86-64-v3, is not available because we intend to build one unified distribution for the x86-64 architecture.Virtual machine models that artificially mask x86-64-v2 CPU features despite host support are comparatively easy to adjust.Our recommendation is based on the following observations: We believe that x86-64-v2 is the appropriate choice for Red Hat Enterprise Linux 9. The remainder of the distribution still does not employ additional CPU features, so those parts of the CPU are essentially dormant.Īs a welcome side-effect of defining the x86-64 microarchitecture levels, we now have a convenient language for discussing the architectural baseline for Linux distributions: We can stay the course and use the original K8 baseline or we can apply one of the three later levels. Each of these approaches applies only to specifically designated blocks of code. So far, we've been able to utilize new CPU features via mechanisms like IFUNCs, function multi-versioning, or loading alternative implementations via dlopen, which could be automated with the ongoing glibc-hwcaps work. There are also significant power requirements when running older hardware. However, due to kernel-driver removals, old hardware (such as systems with first-generation Opteron CPUs) are unlikely to run Red Hat Enterprise Linux in any useful fashion. That decision has held up to and including the latest version of Red Hat Enterprise Linux 8. Historically, the x86_64 Red Hat Enterprise Linux userspace has been built to match the original AMD K8 baseline minus the AMD-specific 3Dnow! parts. These changes are expected to be part of the upcoming 2.33 release of glibc. Patches to augment the glibc dynamic loader with a new mechanism (without the power-set construction) have been incorporated into glibc under the glibc-hwcaps moniker. The upcoming GCC version 11 and LLVM version 12 releases will support them in -march= arguments. We've documented the three levels in detail in the x86-64 psABI supplement. x86-64-v4 includes vector instructions from some of the AVX-512 variants.x86-64-v3 adds vector instructions up to AVX2, MOVBE (for big-endian data access), and additional bit-manipulation instructions.x86-64-v2 brings support (among other things) for vector instructions up to Streaming SIMD Extensions 4.2 (SSE4.2) and Supplemental Streaming SIMD Extensions 3 (SSSE3), the POPCNT instruction (useful for data analysis and bit-fiddling in some data structures), and CMPXCHG16B (a two-word compare-and-swap instruction useful for concurrent algorithms).The three microarchitectures group together CPU features roughly based on hardware release dates: In the summer of 2020, AMD, Intel, Red Hat, and SUSE collaborated to define three x86-64 microarchitecture levels on top of the x86-64 baseline. GCC and glibc disagree on the definition of feature sets, and the glibc selection mechanism is vendor-specific. Until recently, there has been little guidance on what CPU features to assume in optimized libraries. The plethora of choices poses a problem not only for the dynamic linker but also for programmers. We see this especially on the x86 architecture, where many optional features have been added over the years (see the Wikipedia article for the CPUID instruction for a list). However, the power-set construction involved in the library lookup mechanism poorly matches current platforms with a long list of optional CPU features. Originally, this mechanism was designed to support perhaps one or two alternative library implementations, in addition to the default (fallback) implementation that is usually installed in the /usr/lib64 directory. The GNU C Library (glibc) offers a way to load optimized libraries that use additional hardware features that might not be present on all systems. In this article, I discuss a new approach to building the x86-64 variant of Red Hat Enterprise Linux (RHEL) 9 and share Red Hat's recommendation for that build.īackground of the x86-64 microarchitecture levels Programs that use newer CPU instructions might not run on older CPUs. The distribution's default compiler flags are significant for hardware-platform compatibility. One of the most important early decisions when building a Linux distribution is the scope of supported hardware.
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