linux-kernel - Re: [PATCH v1 1/2] KVM: arm64: handle single-stepping trapped instructions

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Date:   Fri, 6 Oct 2017 13:27:22 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Alex Bennée <alex.bennee@...aro.org>,
        julien.thierry@....com, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        christoffer.dall@...aro.org
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 1/2] KVM: arm64: handle single-stepping trapped
 instructions

On 06/10/17 12:39, 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.
> 
> We fake debug.arch.hsr to contain ESR_ELx_EC_SOFTSTP_LOW so QEMU knows
> it was a single-step event (and without altering the userspace ABI).
> 
> Signed-off-by: Alex Bennée <alex.bennee@...aro.org>
> ---
>  arch/arm64/kvm/handle_exit.c | 48 +++++++++++++++++++++++++++++++-------------
>  1 file changed, 34 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 7debb74843a0..c918d291cb58 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -178,6 +178,39 @@ static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)
>  	return arm_exit_handlers[hsr_ec];
>  }
>  
> +/*
> + * When handling traps we need to ensure exit the guest if we
> + * completely emulated the instruction while single-stepping. Stuff to
> + * be emulated in userspace needs to complete that 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);
> +	}
> +
> +	if (handled && (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)) {
> +		handled = 0;
> +		run->exit_reason = KVM_EXIT_DEBUG;
> +		run->debug.arch.hsr = ESR_ELx_EC_SOFTSTP_LOW << ESR_ELx_EC_SHIFT;
> +	}

Doesn't this break an MMIO read? The registers haven't been updated yet,
and the debugger may not see the right thing...

How about something like:

	if (handled && (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)) {
		if (run->exit_reason == KVM_EXIT_MMIO)
			kvm_handle_mmio_return(vcpu, run);
		[...]
	}

Or am I missing something?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...