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Accessing AA64MMFR1_EL1 is expensive in KVM guests, since it is emulated
in the hypervisor. In fact, ARM documentation mentions some feature
registers are not supposed to be accessed frequently by the OS, and
therefore should be emulated for guests [1].
Commit 0388f9c743 ("arm64: mm: Implement
arch_wants_old_prefaulted_pte()") introduced a read of this register in
the page fault path. But, even when the feature of setting faultaround
pages with the old flag is disabled for a given cpu, we are still paying
the cost of checking the register on every pagefault. This results in an
explosion of vmexit events in KVM guests, which directly impacts the
performance of virtualized workloads. For instance, running kernbench
yields a 15% increase in system time solely due to the increased vmexit
cycles.
This patch avoids the extra cost by using the sanitized cached value.
It should be safe to do so, since this register mustn't change for a
given cpu.
[1] https://developer.arm.com/-/media/Arm%20Developer%20Community/PDF/Learn%20the%20Architecture/Armv8-A%20virtualization.pdf?revision=a765a7df-1a00-434d-b241-357bfda2dd31
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20230109151955.8292-1-krisman@suse.de
Signed-off-by: Catalin Marinas <catalin.marinas@arm.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.
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.