[PATCH 4/4] arm64/fpsimd: Document the use of TIF_FOREIGN_FPSTATE by KVM

[Marc Zyngier]

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 at 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