linux-kernel - Re: [PATCH v3 10/12] KVM: arm64: trap nested debug register access

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Date:	Fri, 8 May 2015 18:46:49 +0200
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Alex Bennée <alex.bennee@...aro.org>
Cc:	kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.cs.columbia.edu, marc.zyngier@....com,
	peter.maydell@...aro.org, agraf@...e.de, drjones@...hat.com,
	pbonzini@...hat.com, zhichao.huang@...aro.org,
	jan.kiszka@...mens.com, dahi@...ux.vnet.ibm.com,
	r65777@...escale.com, bp@...e.de, Gleb Natapov <gleb@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 10/12] KVM: arm64: trap nested debug register access

On Thu, May 07, 2015 at 10:07:13AM +0100, Alex Bennée wrote:
> When we are using the hardware registers for guest debug we need to deal
> with the guests access to them. There is already a mechanism for dealing
> with these accesses so we build on top of that.
> 
>   - any access to mdscr_el1 is now stored in the mirror location
>   - access to DBG[WB][CV]R continues to go to guest's context
> 
> There is one register (MDCCINT_EL1) which guest debug doesn't care about
> so this behaves as before.
> 
> Signed-off-by: Alex Bennée <alex.bennee@...aro.org>
> 
> ---
> v3
>   - re-factor for better flow and fall through.
>   - much simpler with debug_ptr (use the guest area as before)
>   - tweak shadow fn to avoid multi-line if
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index a44fb32..7aa3b3a 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -132,7 +132,13 @@ struct kvm_vcpu_arch {
>  	 * here.
>  	 */
>  
> -	/* Guest registers we preserve during guest debugging */
> +	/*
> +	 * Guest registers we preserve during guest debugging.
> +	 *
> +	 * These shadow registers are updated by the kvm_handle_sys_reg
> +	 * trap handler if the guest accesses or updates them while we
> +	 * are using guest debug.
> +	 */
>  	struct {
>  		u32	pstate;
>  		u32	mdscr_el1;
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index 1ab63dd..dc8bca8 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -50,8 +50,7 @@ static void restore_guest_debug_regs(struct kvm_vcpu *vcpu)
>  {
>  	*vcpu_cpsr(vcpu) |=
>  		(vcpu->arch.guest_debug_state.pstate & SPSR_DEBUG_MASK);
> -	vcpu_sys_reg(vcpu, MDSCR_EL1) |=
> -		(vcpu->arch.guest_debug_state.mdscr_el1 & MDSCR_EL1_DEBUG_MASK);
> +	vcpu_sys_reg(vcpu, MDSCR_EL1) = vcpu->arch.guest_debug_state.mdscr_el1;
>  }
>  
>  /**
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index c370b40..95f422f 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -196,11 +196,40 @@ static bool trap_dbgauthstatus_el1(struct kvm_vcpu *vcpu,
>   * - If the dirty bit is set, save guest registers, restore host
>   *   registers and clear the dirty bit. This ensure that the host can
>   *   now use the debug registers.
> + *
> + * We also use this mechanism to set-up the debug registers for guest

s/set-up/set up/

> + * debugging. If this is the case we want to ensure the guest sees

If this is the case, (comma)

> + * the right versions of the registers - even if they are not going to
> + * be effective while guest debug is using HW debug.
> + *
>   */
> +
> +static bool shadow_debug_reg(struct kvm_vcpu *vcpu,
> +			     const struct sys_reg_params *p,
> +			     const struct sys_reg_desc *r)
> +{
> +	/* MDSCR_EL1 */
> +	if (r->reg == MDSCR_EL1) {
> +		u32 *shadow_mdscr_el1 = &vcpu->arch.guest_debug_state.mdscr_el1;
> +
> +		if (p->is_write)
> +			*shadow_mdscr_el1 = *vcpu_reg(vcpu, p->Rt);
> +		else
> +			*vcpu_reg(vcpu, p->Rt) = *shadow_mdscr_el1;
> +
> +		return true;
> +	}
> +
> +	return false;
> +}
> +
>  static bool trap_debug_regs(struct kvm_vcpu *vcpu,
>  			    const struct sys_reg_params *p,
>  			    const struct sys_reg_desc *r)
>  {
> +	if (vcpu->guest_debug && shadow_debug_reg(vcpu, p, r))
> +		return true;
> +

so do we also have a MDSCR_EL1 in sys_regs and one in guest_debug_state
now?

If yes, what are the differences between the two?

>  	if (p->is_write) {
>  		vcpu_sys_reg(vcpu, r->reg) = *vcpu_reg(vcpu, p->Rt);
>  		vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_DIRTY;
> -- 
> 2.3.5
> 
Thanks,
-Christoffer
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