[PATCH] arm: KVM: force execution of HCPTR access on VM exit

[Christoffer Dall]

On Mon, Mar 16, 2015 at 10:59:43AM +0000, Marc Zyngier wrote:
> On VM entry, we disable access to the VFP registers in order to
> perform a lazy save/restore of these registers.
> 
> On VM exit, we restore access, test if we did enable them before,
> and save/restore the guest/host registers if necessary. In this
> sequence, the FPEXC register is always accessed, irrespective
> of the trapping configuration.
> 
> If the guest didn't touch the VFP registers, then the HCPTR access
> has now enabled such access, but we're missing a barrier to ensure
> architectural execution of the new HCPTR configuration. If the HCPTR
> access has been delayed/reordered, the subsequent access to FPEXC
> will cause a trap, which we aren't prepared to handle at all.
> 
> The fix is to introduce a barrier that only takes place if the
> guest hasn't accessed its view of the VFP registers, making
> the access to FPEXC safe.
> 
> Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
> ---
>  arch/arm/kvm/interrupts.S | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/kvm/interrupts.S b/arch/arm/kvm/interrupts.S
> index 79caf79..3ac7aca 100644
> --- a/arch/arm/kvm/interrupts.S
> +++ b/arch/arm/kvm/interrupts.S
> @@ -175,10 +175,13 @@ __kvm_vcpu_return:
>  #ifdef CONFIG_VFPv3
>  	@ Save floating point registers we if let guest use them.
>  	tst	r2, #(HCPTR_TCP(10) | HCPTR_TCP(11))
> -	bne	after_vfp_restore
> +	beq	1f
> +
> +	isb	@ Force execution of HCPTR if we've just reenabled VFP access
> +	b	after_vfp_restore
>  
>  	@ Switch VFP/NEON hardware state to the host's
> -	add	r7, vcpu, #VCPU_VFP_GUEST
> +1:	add	r7, vcpu, #VCPU_VFP_GUEST
>  	store_vfp_state r7
>  	add	r7, vcpu, #VCPU_VFP_HOST
>  	ldr	r7, [r7]
> -- 
> 2.1.4
> 

Reviewed-by: Christoffer Dall <christoffer.dall at linaro.org>