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694d071f8d
(1) During running time of a a VM with numbers of vCPUs, if some vCPUs access the same GPA almost at the same time and the stage-2 mapping of the GPA has not been built yet, as a result they will all cause translation faults. The first vCPU builds the mapping, and the followed ones end up updating the valid leaf PTE. Note that these vCPUs might want different access permissions (RO, RW, RX, RWX, etc.). (2) It's inevitable that we sometimes will update an existing valid leaf PTE in the map path, and we perform break-before-make in this case. Then more unnecessary translation faults could be caused if the *break stage* of BBM is just catched by other vCPUS. With (1) and (2), something unsatisfactory could happen: vCPU A causes a translation fault and builds the mapping with RW permissions, vCPU B then update the valid leaf PTE with break-before-make and permissions are updated back to RO. Besides, *break stage* of BBM may trigger more translation faults. Finally, some useless small loops could occur. We can make some optimization to solve above problems: When we need to update a valid leaf PTE in the map path, let's filter out the case where this update only change access permissions, and don't update the valid leaf PTE here in this case. Instead, let the vCPU enter back the guest and it will exit next time to go through the relax_perms path without break-before-make if it still wants more permissions. Signed-off-by: Yanan Wang <wangyanan55@huawei.com> Signed-off-by: Marc Zyngier <maz@kernel.org> Link: https://lore.kernel.org/r/20210114121350.123684-3-wangyanan55@huawei.com |
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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 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.