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

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Date:   Fri, 6 Oct 2017 13:34:36 +0100
From:   Julien Thierry <julien.thierry@....com>
To:     Marc Zyngier <marc.zyngier@....com>,
        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
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 13:27, Marc Zyngier wrote:
> 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?

If the MMIO was not handled by the kernel, exit_handler will return 0, 
so handled will be false and we won't pretend we have a debug exception 
(but will still return to userland with KVM_EXIT_MMIO).

I think the second patch takes care of properly handling single step for 
userland MMIO.

-- 
Julien Thierry