Re: [PATCH 3/3] arm: KVM: Add VFP lazy switch hooks in Host KVM

[Christoffer Dall <christoffer.dall@linaro.org>]

On Wed, Jun 24, 2015 at 08:30:28PM -0700, Mario Smarduch wrote:
> This patch implements host KVM interface to Hyp mode VFP function to 
> switch out guest and switch in host.
> 
> Signed-off-by: Mario Smarduch <m.smarduch@samsung.com>
> ---
>  arch/arm/kvm/arm.c |   15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index d9631ec..77b41f5 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -105,6 +105,17 @@ void kvm_arch_check_processor_compat(void *rtn)
>  	*(int *)rtn = 0;
>  }
>  
> +/**
> + * kvm_switch_vp_regs() - switch guest/host VFP registers
> + * @vcpu:	pointer to vcpu structure.
> + *
> + * HYP interface functions to save guest and restore host VFP registers

Not sure I understand what you mean to say with this line, how about:

Calls an assembly routine in HYP mode to actually perform the state
save/restore.

However, why do we actually need to do this in hyp mode?  Can't we just
as well do this in SVC mode or are we changing some trap settings here?

> + */
> +static void kvm_switch_fp_regs(struct kvm_vcpu *vcpu)

should probalby be called kvm_vcpu_put_fp_regs

> +{
> +	if (vcpu->arch.vfp_guest_saved == 1)
> +		kvm_call_hyp(__kvm_restore_host_vfp_state, vcpu);
> +}
>  
>  /**
>   * kvm_arch_init_vm - initializes a VM data structure
> @@ -292,6 +303,10 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  
>  void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  {
> +
> +	/* Check if Guest accessed VFP registers */
> +	kvm_switch_fp_regs(vcpu);
> +
>  	/*
>  	 * The arch-generic KVM code expects the cpu field of a vcpu to be -1
>  	 * if the vcpu is no longer assigned to a cpu.  This is used for the
> -- 
> 1.7.9.5

How are we sure that the kernel never touches VFP registers between VCPU
exit and kvm_arch_vcpu_put?  Can a kernel-side memcpy implementation use
the FP regs or something like that?

Thanks,
-Christoffer