linux-kernel - Re: [PATCH v2 2/3] kvm: arm64: handle single-stepping trapped instructions

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Date:   Mon, 13 Nov 2017 11:02:59 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Alex Bennée <alex.bennee@...aro.org>,
        kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, christoffer.dall@...aro.org,
        marc.zyngier@....com
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] kvm: arm64: handle single-stepping trapped
 instructions



On 09/11/17 17:00, Alex Bennée wrote:
> If we are using guest debug to single-step the guest we need to ensure
> we exit after emulating the instruction. This only affects
> instructions completely emulated by the kernel. For userspace emulated
> instructions we need to exit and return to complete the emulation.
> 
> The kvm_arm_handle_step_debug() helper sets up the necessary exit
> state if needed.
> 
> Signed-off-by: Alex Bennée <alex.bennee@...aro.org>

Reviewed-by: Julien Thierry <julien.thierry@....com>

> 
> ---
> v2
>    - use helper from patch 1
>    - if (handled > 0) instead of if (handled) so errors propagate
> ---
>   arch/arm64/kvm/handle_exit.c | 47 +++++++++++++++++++++++++++++++-------------
>   1 file changed, 33 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 7debb74843a0..af1c804742f6 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -178,6 +178,38 @@ static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)
>   	return arm_exit_handlers[hsr_ec];
>   }
>   
> +/*
> + * We may be single-stepping an emulated instruction. If the emulation
> + * has been completed in-kernel we can return to userspace with a
> + * KVM_EXIT_DEBUG, otherwise the userspace needs to complete its
> + * emulation first.
> + */
> +
> +static int handle_trap_exceptions(struct kvm_vcpu *vcpu, struct kvm_run *run)
> +{
> +	int handled;
> +
> +	/*
> +	 * See ARM ARM B1.14.1: "Hyp traps on instructions
> +	 * that fail their condition code check"
> +	 */
> +	if (!kvm_condition_valid(vcpu)) {
> +		kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
> +		handled = 1;
> +	} else {
> +		exit_handle_fn exit_handler;
> +
> +		exit_handler = kvm_get_exit_handler(vcpu);
> +		handled = exit_handler(vcpu, run);
> +	}
> +
> +	/* helper sets exit_reason if we need to return to userspace */
> +	if (handled > 0 && kvm_arm_handle_step_debug(vcpu, run))
> +		handled = 0;
> +
> +	return handled;
> +}
> +
>   /*
>    * Return > 0 to return to guest, < 0 on error, 0 (and set exit_reason) on
>    * proper exit to userspace.
> @@ -185,8 +217,6 @@ static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)
>   int handle_exit(struct kvm_vcpu *vcpu, struct kvm_run *run,
>   		       int exception_index)
>   {
> -	exit_handle_fn exit_handler;
> -
>   	if (ARM_SERROR_PENDING(exception_index)) {
>   		u8 hsr_ec = ESR_ELx_EC(kvm_vcpu_get_hsr(vcpu));
>   
> @@ -214,18 +244,7 @@ int handle_exit(struct kvm_vcpu *vcpu, struct kvm_run *run,
>   		kvm_inject_vabt(vcpu);
>   		return 1;
>   	case ARM_EXCEPTION_TRAP:
> -		/*
> -		 * See ARM ARM B1.14.1: "Hyp traps on instructions
> -		 * that fail their condition code check"
> -		 */
> -		if (!kvm_condition_valid(vcpu)) {
> -			kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
> -			return 1;
> -		}
> -
> -		exit_handler = kvm_get_exit_handler(vcpu);
> -
> -		return exit_handler(vcpu, run);
> +		return handle_trap_exceptions(vcpu, run);
>   	case ARM_EXCEPTION_HYP_GONE:
>   		/*
>   		 * EL2 has been reset to the hyp-stub. This happens when a guest
> 

-- 
Julien Thierry