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Normally when running a guest we do not touch the floating point register state until first use of floating point by the guest, saving the current state and loading the guest state at that point. This has been found to offer a performance benefit in common cases. However currently if SME is active when switching to a guest then we exit streaming mode, disable ZA and invalidate the floating point register state prior to starting the guest. The exit from streaming mode is required for correct guest operation, if we leave streaming mode enabled then many non-SME operations can generate SME traps (eg, SVE operations will become streaming SVE operations). If EL1 leaves CPACR_EL1.SMEN disabled then the host is unable to intercept these traps. This will mean that a SME unaware guest will see SME exceptions which will confuse it. Disabling streaming mode also avoids creating spurious indications of usage of the SME hardware which could impact system performance, especially with shared SME implementations. Document the requirement to exit streaming mode clearly. There is no issue with guest operation caused by PSTATE.ZA so we can defer handling for that until first floating point usage, do so if the register state is not that of the current task and hence has already been saved. We could also do this for the case where the register state is that for the current task however this is very unlikely to happen and would require disproportionate effort so continue to save the state in that case. Saving this state on first use would require that we map and unmap storage for the host version of these registers for use by the hypervisor, taking care to deal with protected KVM and the fact that the host can free or reallocate the backing storage. Given that the strong recommendation is that applications should only keep PSTATE.ZA enabled when the state it enables is in active use it is difficult to see a case where a VMM would wish to do this, it would need to not only be using SME but also running the guest in the middle of SME usage. This can be revisited in the future if a use case does arises, in the interim such tasks will work but experience a performance overhead. This brings our handling of SME more into line with our handling of other floating point state and documents more clearly the constraints we have, especially around streaming mode. Signed-off-by: Mark Brown <broonie@kernel.org> Signed-off-by: Marc Zyngier <maz@kernel.org> Link: https://lore.kernel.org/r/20221214-kvm-arm64-sme-context-switch-v2-3-57ba0082e9ff@kernel.org |
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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.