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09f037aa48
update_permission_bitmask currently does a 128-iteration loop to,
essentially, compute a constant array. Computing the 8 bits in parallel
reduces it to 16 iterations, and is enough to speed it up substantially
because many boolean operations in the inner loop become constants or
simplify noticeably.
Because update_permission_bitmask is actually the top item in the profile
for nested vmexits, this speeds up an L2->L1 vmexit by about ten thousand
clock cycles, or up to 30%:
before after
cpuid 35173 25954
vmcall 35122 27079
inl_from_pmtimer 52635 42675
inl_from_qemu 53604 44599
inl_from_kernel 38498 30798
outl_to_kernel 34508 28816
wr_tsc_adjust_msr 34185 26818
rd_tsc_adjust_msr 37409 27049
mmio-no-eventfd:pci-mem 50563 45276
mmio-wildcard-eventfd:pci-mem 34495 30823
mmio-datamatch-eventfd:pci-mem 35612 31071
portio-no-eventfd:pci-io 44925 40661
portio-wildcard-eventfd:pci-io 29708 27269
portio-datamatch-eventfd:pci-io 31135 27164
(I wrote a small C program to compare the tables for all values of CR0.WP,
CR4.SMAP and CR4.SMEP, and they match).
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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.