Re: [RFC PATCH v2 29/38] KVM: arm64: Support a VM with VHE considering EL0 of the VHE host

[Christoffer Dall]

On Tue, Jul 18, 2017 at 11:58:55AM -0500, Jintack Lim wrote:

nit: The subject is a little hard to understand.

> On VHE systems, EL0 of the host kernel is considered as a part of 'VHE
> host'; The execution of EL0 is affected by system registers set by the
> VHE kernel including the hypervisor. To emulate this for a VM, we use
> the same set of system registers (i.e. shadow registers) for the virtual
> EL2 and EL0 execution.

when the VM sets HCR_EL2.TGE and HCR_EL2.E2H.

> 
> Note that the assumption so far is that a hypervisor in a VM always runs
> in the virtual EL2, and the exception level change from/to the virtual
> EL2 always goes through the host hypervisor. With VHE support for a VM,
> however, the exception level can be changed from EL0 to virtual EL2
> without trapping to the host hypervisor. So, when returning from the VHE
> host mode, set the vcpu mode depending on the physical exception level.

I think there are two changes in this patch which aren't described
properly in the commit message.

First, on entry to a VM that runs in hypervisor context, virtual EL2 or
EL0 with virtual TGE+E2H, we have to either set the physical CPU mode
to EL1 or EL0, for the two cases respectively, where before we would
only ever run in virtual EL2 and would always choose EL1.

Second, on exit from a VM that runs in hypervisor context, virtual EL2 or
EL0 with virtual TGE+E2H, we can no longer assume that we run in virtual
El2, but must consider the hardware state to understand if the exception
from the VM happened from virtual EL2 or from EL0 in the guest
hypervisor's context.

Maybe that helps.

> 
> Signed-off-by: Jintack Lim <jintack.lim@xxxxxxxxxx>
> ---
>  arch/arm64/kvm/context.c | 36 ++++++++++++++++++++++--------------
>  1 file changed, 22 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/arm64/kvm/context.c b/arch/arm64/kvm/context.c
> index f3d3398..39bd92d 100644
> --- a/arch/arm64/kvm/context.c
> +++ b/arch/arm64/kvm/context.c
> @@ -150,16 +150,18 @@ static void flush_shadow_special_regs(struct kvm_vcpu *vcpu)
>  	struct kvm_cpu_context *ctxt = &vcpu->arch.ctxt;
>  
>  	ctxt->hw_pstate = *vcpu_cpsr(vcpu) & ~PSR_MODE_MASK;
> -	/*
> -	 * We can emulate the guest's configuration of which
> -	 * stack pointer to use when executing in virtual EL2 by
> -	 * using the equivalent feature in EL1 to point to
> -	 * either the EL1 or EL0 stack pointer.
> -	 */
> -	if ((*vcpu_cpsr(vcpu) & PSR_MODE_MASK) == PSR_MODE_EL2h)
> -		ctxt->hw_pstate |= PSR_MODE_EL1h;
> -	else
> -		ctxt->hw_pstate |= PSR_MODE_EL1t;
> +	if (vcpu_mode_el2(vcpu)) {
> +		/*
> +		 * We can emulate the guest's configuration of which
> +		 * stack pointer to use when executing in virtual EL2 by
> +		 * using the equivalent feature in EL1 to point to
> +		 * either the EL1 or EL0 stack pointer.
> +		 */
> +		if ((*vcpu_cpsr(vcpu) & PSR_MODE_MASK) == PSR_MODE_EL2h)
> +			ctxt->hw_pstate |= PSR_MODE_EL1h;
> +		else
> +			ctxt->hw_pstate |= PSR_MODE_EL1t;
> +	}

This looks funny, because now you don't set a mode unless
vcpu_mode_el2(vcpu) is true, which happens to work because the only
other choice is PSR_MODE_EL0t which happens to be 0.

>  
>  	ctxt->hw_sys_regs = ctxt->shadow_sys_regs;
>  	ctxt->hw_sp_el1 = vcpu_el2_sreg(vcpu, SP_EL2);
> @@ -182,8 +184,14 @@ static void sync_shadow_special_regs(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_cpu_context *ctxt = &vcpu->arch.ctxt;
>  
> -	*vcpu_cpsr(vcpu) &= PSR_MODE_MASK;
> -	*vcpu_cpsr(vcpu) |= ctxt->hw_pstate & ~PSR_MODE_MASK;
> +	*vcpu_cpsr(vcpu) = ctxt->hw_pstate;
> +	*vcpu_cpsr(vcpu) &= ~PSR_MODE_MASK;
> +	/* Set vcpu exception level depending on the physical EL */
> +	if ((ctxt->hw_pstate & PSR_MODE_MASK) == PSR_MODE_EL0t)
> +		*vcpu_cpsr(vcpu) |= PSR_MODE_EL0t;
> +	else
> +		*vcpu_cpsr(vcpu) |= PSR_MODE_EL2h;
> +

don't you need to distinguish between PSR_MODE_EL2h and PSR_MODE_EL2t
here?



>  	vcpu_el2_sreg(vcpu, SP_EL2) = ctxt->hw_sp_el1;
>  	vcpu_el2_sreg(vcpu, ELR_EL2) = ctxt->hw_elr_el1;
>  	vcpu_el2_sreg(vcpu, SPSR_EL2) = ctxt->hw_spsr_el1;
> @@ -218,7 +226,7 @@ void kvm_arm_setup_shadow_state(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_cpu_context *ctxt = &vcpu->arch.ctxt;
>  
> -	if (unlikely(vcpu_mode_el2(vcpu))) {
> +	if (unlikely(is_hyp_ctxt(vcpu))) {
>  		flush_shadow_special_regs(vcpu);
>  		flush_shadow_el1_sysregs(vcpu);
>  		flush_shadow_non_trap_el1_state(vcpu);
> @@ -236,7 +244,7 @@ void kvm_arm_setup_shadow_state(struct kvm_vcpu *vcpu)
>   */
>  void kvm_arm_restore_shadow_state(struct kvm_vcpu *vcpu)
>  {
> -	if (unlikely(vcpu_mode_el2(vcpu))) {
> +	if (unlikely(is_hyp_ctxt(vcpu))) {
>  		sync_shadow_special_regs(vcpu);
>  		sync_shadow_non_trap_el1_state(vcpu);
>  	} else
> -- 
> 1.9.1
> 

Thanks,
-Christoffer