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POWER9 introduces a new mode for the decrementer register, called large decrementer mode, in which the decrementer counter is 56 bits wide rather than 32, and reads are sign-extended rather than zero-extended. For the decrementer, this new mode is optional and controlled by a bit in the LPCR. The hypervisor decrementer (HDEC) is 56 bits wide on POWER9 and has no mode control. Since KVM code reads and writes the decrementer and hypervisor decrementer registers in a few places, it needs to be aware of the need to treat the decrementer value as a 64-bit quantity, and only do a 32-bit sign extension when large decrementer mode is not in effect. Similarly, the HDEC should always be treated as a 64-bit quantity on POWER9. We define a new EXTEND_HDEC macro to encapsulate the feature test for POWER9 and the sign extension. To enable the sign extension to be removed in large decrementer mode, we test the LPCR_LD bit in the host LPCR image stored in the struct kvm for the guest. If is set then large decrementer mode is enabled and the sign extension should be skipped. This is partly based on an earlier patch by Oliver O'Halloran. Cc: stable@vger.kernel.org # v4.10+ Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.