linux-kernel - Re: [RFC 08/55] KVM: arm64: Set virtual EL2 context depending on the guest exception level

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Date:   Wed, 22 Feb 2017 12:14:45 +0100
From:   Christoffer Dall <cdall@...aro.org>
To:     Jintack Lim <jintack@...columbia.edu>
Cc:     christoffer.dall@...aro.org, marc.zyngier@....com,
        pbonzini@...hat.com, rkrcmar@...hat.com, linux@...linux.org.uk,
        catalin.marinas@....com, will.deacon@....com,
        vladimir.murzin@....com, suzuki.poulose@....com,
        mark.rutland@....com, james.morse@....com,
        lorenzo.pieralisi@....com, kevin.brodsky@....com,
        wcohen@...hat.com, shankerd@...eaurora.org, geoff@...radead.org,
        andre.przywara@....com, eric.auger@...hat.com,
        anna-maria@...utronix.de, shihwei@...columbia.edu,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 08/55] KVM: arm64: Set virtual EL2 context depending on the
 guest exception level

On Mon, Jan 09, 2017 at 01:24:04AM -0500, Jintack Lim wrote:
> From: Christoffer Dall <christoffer.dall@...aro.org>
> 
> Set up virutal EL2 context to hardware if the guest exception level is
> EL2.
> 
> Signed-off-by: Christoffer Dall <christoffer.dall@...aro.org>
> Signed-off-by: Jintack Lim <jintack@...columbia.edu>
> ---
>  arch/arm64/kvm/context.c | 32 ++++++++++++++++++++++++++------
>  1 file changed, 26 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/kvm/context.c b/arch/arm64/kvm/context.c
> index 320afc6..acb4b1e 100644
> --- a/arch/arm64/kvm/context.c
> +++ b/arch/arm64/kvm/context.c
> @@ -25,10 +25,25 @@
>  void kvm_arm_setup_shadow_state(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_cpu_context *ctxt = &vcpu->arch.ctxt;
> +	if (unlikely(vcpu_mode_el2(vcpu))) {
> +		ctxt->hw_pstate = *vcpu_cpsr(vcpu) & ~PSR_MODE_MASK;
>  
> -	ctxt->hw_pstate = *vcpu_cpsr(vcpu);
> -	ctxt->hw_sys_regs = ctxt->sys_regs;
> -	ctxt->hw_sp_el1 = ctxt->gp_regs.sp_el1;
> +		/*
> +		 * We emulate virtual EL2 mode in hardware EL1 mode using the
> +		 * same stack pointer mode as the guest expects.
> +		 */

I think this comment should either be deleted or explain why this works
as opposed to stating the obvious.  How about:

		/*
		 * 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;
> +
> +		ctxt->hw_sys_regs = ctxt->shadow_sys_regs;
> +		ctxt->hw_sp_el1 = ctxt->el2_regs[SP_EL2];
> +	} else {
> +		ctxt->hw_pstate = *vcpu_cpsr(vcpu);
> +		ctxt->hw_sys_regs = ctxt->sys_regs;
> +		ctxt->hw_sp_el1 = ctxt->gp_regs.sp_el1;
> +	}
>  }
>  
>  /**
> @@ -38,9 +53,14 @@ void kvm_arm_setup_shadow_state(struct kvm_vcpu *vcpu)
>  void kvm_arm_restore_shadow_state(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_cpu_context *ctxt = &vcpu->arch.ctxt;
> -
> -	*vcpu_cpsr(vcpu) = ctxt->hw_pstate;
> -	ctxt->gp_regs.sp_el1 = ctxt->hw_sp_el1;
> +	if (unlikely(vcpu_mode_el2(vcpu))) {
> +		*vcpu_cpsr(vcpu) &= PSR_MODE_MASK;
> +		*vcpu_cpsr(vcpu) |= ctxt->hw_pstate & ~PSR_MODE_MASK;
> +		ctxt->el2_regs[SP_EL2] = ctxt->hw_sp_el1;
> +	} else {
> +		*vcpu_cpsr(vcpu) = ctxt->hw_pstate;
> +		ctxt->gp_regs.sp_el1 = ctxt->hw_sp_el1;
> +	}
>  }
>  
>  void kvm_arm_init_cpu_context(kvm_cpu_context_t *cpu_ctxt)
> -- 
> 1.9.1
> 
>