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The FRED RSP0 MSR points to the top of the kernel stack for user level event delivery. As this is the task stack it needs to be updated when a task is scheduled in. The update is done at context switch. That means it's also done when switching to kernel threads, which is pointless as those never go out to user space. For KVM threads this means there are two writes to FRED_RSP0 as KVM has to switch to the guest value before VMENTER. Defer the update to the exit to user space path and cache the per CPU FRED_RSP0 value, so redundant writes can be avoided. Provide fred_sync_rsp0() for KVM to keep the cache in sync with the actual MSR value after returning from guest to host mode. [ tglx: Massage change log ] Suggested-by: Sean Christopherson <seanjc@google.com> Suggested-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Xin Li (Intel) <xin@zytor.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lore.kernel.org/all/20240822073906.2176342-4-xin@zytor.com
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 reStructuredText 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.