From: Marc Zyngier Date: Thu, 21 Oct 2021 14:33:22 +0000 (+0100) Subject: arm64/fpsimd: Document the use of TIF_FOREIGN_FPSTATE by KVM X-Git-Tag: v5.17-rc1~15^2~96^2~8^2 X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=31aa126de88e1e9f562f708a9b0ec5917ba97dce;p=linux-block.git 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: Mark Brown Signed-off-by: Marc Zyngier --- 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.