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fe769e6c1f
It recently appeared that, when running VHE, there is a notable
difference between using CNTKCTL_EL1 and CNTHCTL_EL2, despite what
the architecture documents:
- When accessed from EL2, bits [19:18] and [16:10] of CNTKCTL_EL1 have
the same assignment as CNTHCTL_EL2
- When accessed from EL1, bits [19:18] and [16:10] are RES0
It is all OK, until you factor in NV, where the EL2 guest runs at EL1.
In this configuration, CNTKCTL_EL11 doesn't trap, nor ends up in
the VNCR page. This means that any write from the guest affecting
CNTHCTL_EL2 using CNTKCTL_EL1 ends up losing some state. Not good.
The fix it obvious: don't use CNTKCTL_EL1 if you want to change bits
that are not part of the EL1 definition of CNTKCTL_EL1, and use
CNTHCTL_EL2 instead. This doesn't change anything for a bare-metal OS,
and fixes it when running under NV. The NV hypervisor will itself
have to work harder to merge the two accessors.
Note that there is a pending update to the architecture to address
this issue by making the affected bits UNKNOWN when CNTKCTL_EL1 is
used from EL2 with VHE enabled.
Fixes:
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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.