86833aec44
The current CR3 handling for kernel page table isolation in the paranoid return paths which are relevant for #NMI, #MCE, #VC, #DB and #DF is unconditionally writing CR3 with the value retrieved on exception entry. In the vast majority of cases when returning to the kernel this is a pointless exercise because CR3 was not modified on exception entry. The only situation where this is necessary is when the exception interrupts a entry from user before switching to kernel CR3 or interrupts an exit to user after switching back to user CR3. As CR3 writes can be expensive on some systems this becomes measurable overhead with high frequency #NMIs such as perf. Avoid this overhead by checking the CR3 value, which was saved on entry, and write it back to CR3 only when it us a user CR3. -----BEGIN PGP SIGNATURE----- iQJHBAABCgAxFiEEQp8+kY+LLUocC4bMphj1TA10mKEFAmXvTXYTHHRnbHhAbGlu dXRyb25peC5kZQAKCRCmGPVMDXSYoYMED/40YXFa0si5/9LRh/LSYglxVe/RaXCn 3oU19oWFRxdHCCLYHeQdlQGrpugM773X+4EC1dE92QpYjFnuLhl5H10h3t2e+3Uw Q2VoWEo95FuJ2v7nqex7p2pglOvNjT2VBBlcFFdhqxiC1FCupXvU17nCcLeBsPkj wbY2Sq4DxPDoWhWMNK2jhCQNVyYYluJERylS5+j0CK8vhQghq1N1WjcB6tQiAYsa 7nXz2ZJeGF0jnvLanyhAVSHDKU7QOMO3zkQpaaMlGQ9izawupe5/Gbi8ouFieCh+ xoLnGo1sgtMOXInnYaJnCiwuc+WiVN3d83aO/s7NZi8ZF60ib72xhzsRip2Cu4aV kBtJaCVLFItQZ81HRSBABj6s9MLphHVm4AaOCvCIxK0ib5KDFaWy3tZpwTU4dvwX rcwKsQrSLlOOD5zqO5dZn+HX6hK2lsNeTPLfcKVqARGn5S9fITzYbUMlkhO/FGaj ZhIgadH8+rXwFDbgS6CGbVYKtM6Ncf/VBGFfE7tEOUQVUmLws3pdLiWo6I2QTGtw fCAeF9uYmvhtiKk0e2jotZdbAg6HP2XTQSZfBxQpRgY6AnYW+XyDezcN0X1eNMJC lmNC72WYxURHZUoOIxiiVzDS9kz7YTUo3pBHFrpQlNqGTqP8r+tAhUyou16yDK/0 2G9Mms/85u89MQ== =UcMe -----END PGP SIGNATURE----- Merge tag 'x86-entry-2024-03-11' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip Pull x86 entry update from Thomas Gleixner: "A single update for the x86 entry code: The current CR3 handling for kernel page table isolation in the paranoid return paths which are relevant for #NMI, #MCE, #VC, #DB and #DF is unconditionally writing CR3 with the value retrieved on exception entry. In the vast majority of cases when returning to the kernel this is a pointless exercise because CR3 was not modified on exception entry. The only situation where this is necessary is when the exception interrupts a entry from user before switching to kernel CR3 or interrupts an exit to user after switching back to user CR3. As CR3 writes can be expensive on some systems this becomes measurable overhead with high frequency #NMIs such as perf. Avoid this overhead by checking the CR3 value, which was saved on entry, and write it back to CR3 only when it is a user CR3" * tag 'x86-entry-2024-03-11' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: x86/entry: Avoid redundant CR3 write on paranoid returns
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 Restructured Text 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.