Linux mainline fork with MSM8998 patches | https://mainline.space | Currently supported devices:
OnePlus 5/5T, Xiaomi Mi 6, F(x)tec Pro¹ (2019 QX1000 model) & Sony Xperia XZ Premium (UNTESTED!)
aaae8c7900
Dependent-threads mode is the normal KVM mode for pre-POWER9 SMT processors, where all threads in a core (or subcore) would run the same partition at the same time, or they would run the host. This design was mandated by MMU state that is shared between threads in a processor, so the synchronisation point is in hypervisor real-mode that has essentially no shared state, so it's safe for multiple threads to gather and switch to the correct mode. It is implemented by having the host unplug all secondary threads and always run in SMT1 mode, and host QEMU threads essentially represent virtual cores that wake these secondary threads out of unplug when the ioctl is called to run the guest. This happens via a side-path that is mostly invisible to the rest of the Linux host and the secondary threads still appear to be unplugged. POWER9 / ISA v3.0 has a more flexible MMU design that is independent per-thread and allows a much simpler KVM implementation. Before the new "P9 fast path" was added that began to take advantage of this, POWER9 support was implemented in the existing path which has support to run in the dependent threads mode. So it was not much work to add support to run POWER9 in this dependent threads mode. The mode is not required by the POWER9 MMU (although "mixed-mode" hash / radix MMU limitations of early processors were worked around using this mode). But it is one way to run SMT guests without running different guests or guest and host on different threads of the same core, so it could avoid or reduce some SMT attack surfaces without turning off SMT entirely. This security feature has some real, if indeterminate, value. However the old path is lagging in features (nested HV), and with this series the new P9 path adds remaining missing features (radix prefetch bug and hash support, in later patches), so POWER9 dependent threads mode support would be the only remaining reason to keep that code in and keep supporting POWER9/POWER10 in the old path. So here we make the call to drop this feature. Remove dependent threads mode support for POWER9 and above processors. Systems can still achieve this security by disabling SMT entirely, but that would generally come at a larger performance cost for guests. Signed-off-by: Nicholas Piggin <npiggin@gmail.com> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Link: https://lore.kernel.org/r/20210528090752.3542186-23-npiggin@gmail.com |
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| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
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.