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On systems that advertise ICH_VTR_EL2.SEIS, we trap all GICv3 sysreg accesses from the guest. From a performance perspective, this is OK as long as the guest doesn't hammer the GICv3 CPU interface. In most cases, this is fine, unless the guest actively uses priorities and switches PMR_EL1 very often. Which is exactly what happens when a Linux guest runs with irqchip.gicv3_pseudo_nmi=1. In these condition, the performance plumets as we hit PMR each time we mask/unmask interrupts. Not good. There is however an opportunity for improvement. Careful reading of the architecture specification indicates that the only GICv3 sysreg belonging to the common group (which contains the SGI registers, PMR, DIR, CTLR and RPR) that is allowed to generate a SError is DIR. Everything else is safe. It is thus possible to substitute the trapping of all the common group with just that of DIR if it supported by the implementation. Yes, that's yet another optional bit of the architecture. So let's just do that, as it leads to some impressive result on the M1: Without this change: bash-5.1# /host/home/maz/hackbench 100 process 1000 Running with 100*40 (== 4000) tasks. Time: 56.596 With this change: bash-5.1# /host/home/maz/hackbench 100 process 1000 Running with 100*40 (== 4000) tasks. Time: 8.649 which is a pretty convincing result. Signed-off-by: Marc Zyngier <maz@kernel.org> Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com> Link: https://lore.kernel.org/r/20211010150910.2911495-4-maz@kernel.org
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.