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The OPAL call wrapper gets interrupt disabling wrong. It disables interrupts just by clearing MSR[EE], which has two problems: - It doesn't call into the IRQ tracing subsystem, which means tracing across OPAL calls does not always notice IRQs have been disabled. - It doesn't go through the IRQ soft-mask code, which causes a minor bug. MSR[EE] can not be restored by saving the MSR then clearing MSR[EE], because a racing interrupt while soft-masked could clear MSR[EE] between the two steps. This can cause MSR[EE] to be incorrectly enabled when the OPAL call returns. Fortunately that should only result in another masked interrupt being taken to disable MSR[EE] again, but it's a bit sloppy. The existing code also saves MSR to PACA, which is not re-entrant if there is a nested OPAL call from different MSR contexts, which can happen these days with SRESET interrupts on bare metal. To fix these issues, move the tracing and IRQ handling code to C, and call into asm just for the low level call when everything is ready to go. Save the MSR on stack rather than PACA. Performance cost is kept to a minimum with a few optimisations: - The endian switch upon return is combined with the MSR restore, which avoids an expensive context synchronizing operation for LE kernels. This makes up for the additional mtmsrd to enable interrupts with local_irq_enable(). - blr is now used to return from the opal_* functions that are called as C functions, to avoid link stack corruption. This requires a skiboot fix as well to keep the call stack balanced. A NULL call is more costly after this, (410ns->430ns on POWER9), but OPAL calls are generally not performance critical at this scale. Signed-off-by: Nicholas Piggin <npiggin@gmail.com> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> |
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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.