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For backwards compatibility reasons, when a signal return occurs which restores SVE state, the effective lower 128 bits of each of the SVE vector registers are restored from the corresponding FPSIMD vector register in the FPSIMD signal frame, overriding the values in the SVE signal frame. This is intended to be the case regardless of streaming mode. To make this happen, restore_sve_fpsimd_context() uses fpsimd_update_current_state() to merge the lower 128 bits from the FPSIMD signal frame into the SVE register state. Unfortunately, fpsimd_update_current_state() performs this merging dependent upon TIF_SVE, which is not always correct for streaming SVE register state: * When restoring non-streaming SVE register state there is no observable problem, as the signal return code configures TIF_SVE and the saved fp_type to match before calling fpsimd_update_current_state(), which observes either: - TIF_SVE set AND fp_type == FP_STATE_SVE - TIF_SVE clear AND fp_type == FP_STATE_FPSIMD * On systems which have SME but not SVE, TIF_SVE cannot be set. Thus the merging will never happen for the streaming SVE register state. * On systems which have SVE and SME, TIF_SVE can be set and cleared independently of PSTATE.SM. Thus the merging may or may not happen for streaming SVE register state. As TIF_SVE can be cleared non-deterministically during syscalls (including at the start of sigreturn()), the merging may occur non-deterministically from the perspective of userspace. This logic has been broken since its introduction in commit: |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.