[PATCH 4/4] arm64/fpsimd: Document the use of TIF_FOREIGN_FPSTATE by KVM
From: Marc Zyngier <maz@kernel.org>
Date: 2021-10-21 15:16:14
Also in:
kvm, kvmarm
Subsystem:
arm64 port (aarch64 architecture), the rest · Maintainers:
Catalin Marinas, Will Deacon, Linus Torvalds
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. Signed-off-by: Marc Zyngier <maz@kernel.org> --- arch/arm64/kernel/fpsimd.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index ff4962750b3d..65af2ed64c6a 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c@@ -78,7 +78,10 @@ * 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 *or* the state of the corresponding KVM vcpu if userspace is behaving + * as a VMM and that the vcpu has used FP during its last run. In the latter + * case, KVM will set TIF_FOREIGN_FPSTATE on kvm_vcpu_put(). For all intents + * and purposes, the vcpu FP state is treated identically to userspace's. * * 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.
--
2.30.2
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