OSXSAVE

OSXSAVE is a processor feature flag that indicates the operating system supports saving and restoring extended processor states across context switches using the XSAVE/XRSTOR instructions and the XSTATE mechanism. It is reported by the processor’s CPUID instruction, and its presence allows the kernel to manage the full set of architectural and microarchitectural state that may be enabled for a program or thread.

When OSXSAVE is set, the operating system can use XSAVE, XRSTOR and related instructions to save and restore not only the traditional x87 FPU and SSE state but also additional state components such as AVX, AVX-512, and other future extensions. The components that may be saved are controlled by the XCR0 register, whose bits indicate which state classes the OS has enabled for the current process. The actual set of components supported depends on the processor’s hardware features and what the OS has chosen to enable.

In practice, the OS allocates per-task storage for the XSTATE region and uses XSAVE/XRSTOR during context switches to persist and restore the enabled state. This enables efficient context switching for workloads that rely on wide vector registers and other extended state. If OSXSAVE is not available or not enabled by the OS, the kernel must rely on legacy saving mechanisms and may not preserve extended state beyond the basics, potentially impacting performance for modern workloads and virtualization.

In summary, OSXSAVE signifies that the operating system can leverage the XSTATE framework to save and restore extended processor state, enabling efficient use of advanced vector and other state components.