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UPSTREAM: arm64/fpsimd: Document the use of TIF_FOREIGN_FPSTATE by KVM

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The bit of documentation that talks about TIF_FOREIGN_FPSTATE
does not mention the ungodly tricks that KVM plays with this flag.

Try and document this for the posterity.

Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
(cherry picked from commit 31aa126de8)
Signed-off-by: Will Deacon <willdeacon@google.com>
Bug: 233587962
Bug: 233588291
Change-Id: Iec0b06e35ad286d6bcea15745f2a1b160ff967cc
This commit is contained in:
Marc Zyngier
2021-10-21 15:33:22 +01:00
committed by Will Deacon
parent 68f75f2392
commit d3af34fbf4
1 changed files with 5 additions and 1 deletions
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6
arch/arm64/kernel/fpsimd.c
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@@ -78,7 +78,11 @@
* indicate whether or not the userland FPSIMD state of the current task is
* present in the registers. The flag is set unless the FPSIMD registers of this
* CPU currently contain the most recent userland FPSIMD state of the current
* task.
* task. If the task is behaving as a VMM, then this is will be managed by
* KVM which will clear it to indicate that the vcpu FPSIMD state is currently
* loaded on the CPU, allowing the state to be saved if a FPSIMD-aware
* softirq kicks in. Upon vcpu_put(), KVM will save the vcpu FP state and
* flag the register state as invalid.
*
* In order to allow softirq handlers to use FPSIMD, kernel_neon_begin() may
* save the task's FPSIMD context back to task_struct from softirq context.