Commit 31aa126d authored by Marc Zyngier's avatar Marc Zyngier
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arm64/fpsimd: Document the use of TIF_FOREIGN_FPSTATE by KVM 



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: default avatarMark Brown <broonie@kernel.org>
Signed-off-by: default avatarMarc Zyngier <maz@kernel.org>
parent bee14bca

arch/arm64/kernel/fpsimd.c

+5 −1
Original line number Diff line number Diff line
@@ -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.