linux-kernel - Re: [RFC PATCH] KVM: arm64: don't single-step for non-emulated faults

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Message-ID: <20181107180120.urnvkcrkh46ytsdb@lakrids.cambridge.arm.com>
Date:   Wed, 7 Nov 2018 18:01:20 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Alex Bennée <alex.bennee@...aro.org>
Cc:     kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, christoffer.dall@...aro.org,
        marc.zyngier@....com, Christoffer Dall <christoffer.dall@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] KVM: arm64: don't single-step for non-emulated faults

On Wed, Nov 07, 2018 at 05:10:31PM +0000, Alex Bennée wrote:
> Not all faults handled by handle_exit are instruction emulations. For
> example a ESR_ELx_EC_IABT will result in the page tables being updated
> but the instruction that triggered the fault hasn't actually executed
> yet. We use the simple heuristic of checking for a changed PC before
> seeing if kvm_arm_handle_step_debug wants to claim we stepped an
> instruction.
> 
> Signed-off-by: Alex Bennée <alex.bennee@...aro.org>
> ---
>  arch/arm64/kvm/handle_exit.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index e5e741bfffe1..b8252e72f882 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -214,6 +214,7 @@ static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)
>  static int handle_trap_exceptions(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  {
>  	int handled;
> +        unsigned long old_pc = *vcpu_pc(vcpu);
>  
>  	/*
>  	 * See ARM ARM B1.14.1: "Hyp traps on instructions
> @@ -233,7 +234,8 @@ static int handle_trap_exceptions(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  	 * kvm_arm_handle_step_debug() sets the exit_reason on the kvm_run
>  	 * structure if we need to return to userspace.
>  	 */
> -	if (handled > 0 && kvm_arm_handle_step_debug(vcpu, run))
> +	if (handled > 0 && *vcpu_pc(vcpu) != old_pc &&

This doesn't work if the emulation is equivalent to a branch-to-self, so
I don't think that we want to do this.

When are we failing to advance the single-step state machine correctly?

Thanks,
Mark.

> +	    kvm_arm_handle_step_debug(vcpu, run))
>  		handled = 0;
>  
>  	return handled;
> -- 
> 2.17.1
>