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[v3,6/6] arm64/fpsimd: Document the use of TIF_FOREIGN_FPSTATE by KVM

Message ID 20211201120436.389756-7-maz@kernel.org (mailing list archive)
State New, archived
Headers show
Series KVM: arm64: Rework FPSIMD/SVE tracking | expand

Commit Message

Marc Zyngier Dec. 1, 2021, 12:04 p.m. UTC 
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>
---
 arch/arm64/kernel/fpsimd.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
diff mbox series

Patch

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index fa244c426f61..6fb361e8bed8 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -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.