Re: [PATCH] Revert "x86/fpu: Refine and simplify the magic number check during signal return"

[Chang S. Bae]

On 5/1/2026 11:44 AM, Andrei Vagin wrote:
> I've been thinking about this more, and I believe the claim that XSAVE
> offsets can differ across CPUs for the same feature is inaccurate. The
> XSAVE standard format uses fixed offsets specifically to allow migration
> between different CPU generations. If a feature exists on both the
> source and destination CPUs, its data resides at the exact same byte
> offset.

There is commit ba386777a30b ("x86/elf: Add a new FPU buffer layout info 
to x86 core files") for this reason:

    ...
    The XSAVE layouts of modern AMD and Intel CPUs differ, especially
    since Memory Protection Keys and the AVX-512 features have been
    inculcated into the AMD CPUs.

    Since AMD never adopted (and hence never left room in the XSAVE
    layout for) the Intel MPX feature, tools like GDB had assumed a
    fixed XSAVE layout matching that of Intel (based on the XCR0 mask).

    Hence, core dumps from AMD CPUs didn't match the known size for the
    XCR0 mask. This resulted in GDB and other tools not being able to
    access the values of the AVX-512 and PKRU registers on AMD CPUs.
    ...

Thanks,
Chang