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Enlightened MSR-Bitmap is a natural extension of Enlightened VMCS: Hyper-V Top Level Functional Specification states: "The L1 hypervisor may collaborate with the L0 hypervisor to make MSR accesses more efficient. It can enable enlightened MSR bitmaps by setting the corresponding field in the enlightened VMCS to 1. When enabled, the L0 hypervisor does not monitor the MSR bitmaps for changes. Instead, the L1 hypervisor must invalidate the corresponding clean field after making changes to one of the MSR bitmaps." I reached out to Hyper-V team for additional details and I got the following information: "Current Hyper-V implementation works as following: If the enlightened MSR bitmap is not enabled: - All MSR accesses of L2 guests cause physical VM-Exits If the enlightened MSR bitmap is enabled: - Physical VM-Exits for L2 accesses to certain MSRs (currently FS_BASE, GS_BASE and KERNEL_GS_BASE) are avoided, thus making these MSR accesses faster." I tested my series with a tight rdmsrl loop in L2, for KERNEL_GS_BASE the results are: Without Enlightened MSR-Bitmap: 1300 cycles/read With Enlightened MSR-Bitmap: 120 cycles/read Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com> Tested-by: Lan Tianyu <Tianyu.Lan@microsoft.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.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 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.