linux-kernel - Re: [PATCH v1 2/2] kvm: arm64: handle single-step of userspace mmio instructions

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Message-ID: <87po9r9zau.fsf@linaro.org>
Date:   Fri, 13 Oct 2017 10:23:21 +0100
From:   Alex Bennée <alex.bennee@...aro.org>
To:     Christoffer Dall <cdall@...aro.org>
Cc:     Julien Thierry <julien.thierry@....com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        christoffer.dall@...aro.org, marc.zyngier@....com,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 2/2] kvm: arm64: handle single-step of userspace mmio instructions


Christoffer Dall <cdall@...aro.org> writes:

> On Fri, Oct 06, 2017 at 02:45:35PM +0100, Alex Bennée wrote:
>>
>> Julien Thierry <julien.thierry@....com> writes:
>>
>> > On 06/10/17 12:39, Alex Bennée wrote:
>> >> The system state of KVM when using userspace emulation is not complete
>> >> until we return into KVM_RUN. To handle mmio related updates we wait
>> >> until they have been committed and then schedule our KVM_EXIT_DEBUG.
>> >>
>> >> I've introduced a new function kvm_arm_maybe_return_debug() to wrap up
>> >> the differences between arm/arm64 which is currently null for arm.
>> >>
>> >> Signed-off-by: Alex Bennée <alex.bennee@...aro.org>
>> >> ---
>> >>   arch/arm/include/asm/kvm_host.h   |  2 ++
>> >>   arch/arm64/include/asm/kvm_host.h |  1 +
>> >>   arch/arm64/kvm/debug.c            | 21 +++++++++++++++++++++
>> >>   arch/arm64/kvm/handle_exit.c      |  9 +++------
>> >>   virt/kvm/arm/arm.c                |  2 +-
>> >>   virt/kvm/arm/mmio.c               |  3 ++-
>> >>   6 files changed, 30 insertions(+), 8 deletions(-)
>> >>
>> >> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
>> >> index 4a879f6ff13b..aec943f6d123 100644
>> >> --- a/arch/arm/include/asm/kvm_host.h
>> >> +++ b/arch/arm/include/asm/kvm_host.h
>> >> @@ -285,6 +285,8 @@ static inline void kvm_arm_init_debug(void) {}
>> >>   static inline void kvm_arm_setup_debug(struct kvm_vcpu *vcpu) {}
>> >>   static inline void kvm_arm_clear_debug(struct kvm_vcpu *vcpu) {}
>> >>   static inline void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu) {}
>> >> +static inline int  kvm_arm_maybe_return_debug(struct kvm_vcpu *vcpu,
>> >> +						struct kvm_run *run) {}
>> >>
>> >
>> > This function should return 1.
>>
>> So I did ponder making this a bool, returning true if we need to exit
>> and testing in v/k/a/arm.c exit leg rather than in the mmio handler.
>>
>> At the moment it mirrors the existing exit logic which follows -1 err, 0
>> return, >0 handled. But as I mentioned in the cover letter this fell
>> down a bit when dealing with the mmio case.
>>
>> >
>> >>   int kvm_arm_vcpu_arch_set_attr(struct kvm_vcpu *vcpu,
>> >>   			       struct kvm_device_attr *attr);
>> >> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> >> index e923b58606e2..fa67d21662f6 100644
>> >> --- a/arch/arm64/include/asm/kvm_host.h
>> >> +++ b/arch/arm64/include/asm/kvm_host.h
>> >> @@ -369,6 +369,7 @@ void kvm_arm_init_debug(void);
>> >>   void kvm_arm_setup_debug(struct kvm_vcpu *vcpu);
>> >>   void kvm_arm_clear_debug(struct kvm_vcpu *vcpu);
>> >>   void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu);
>> >> +int  kvm_arm_maybe_return_debug(struct kvm_vcpu *vcpu, struct kvm_run *run);
>> >
>> > I feel the name could be a little bit more explicit:
>> >
>> > kvm_arm_trap_need_step_debug, kvm_arm_trap_step_return_debug,
>> > kvm_arm_trap_need_return_debug.
>>
>> I wanted to keep the debug suffix so that's fine although I'm not so
>> sure trap is correct because on the tail end of mmio emulation are we
>> still trapping?
>>
>> Maybe kvm_arm_step_emulated_debug?
>
> I think you should name it:
>
> kvm_arm_should_complete_emulated_instr_debug() - or something better -

Naming is hard :-/

> and call it directly from kvm_arch_vcpu_ioctl_run, so that it becomes:
>
> 	ret = kvm_handle_mmio_return(vcpu, vcpu->run);
> 	if (ret)
> 		return ret;
> 	ret = kvm_arm_should_complete_emulated_instr_debug(vcpu);
> 	if (ret)
> 		return ret;

This runs into the problem of slightly different ret semantics for here
and in handle_exit. Maybe just having a bool response and:

    if (kvm_arm_should_complete_emulated_instr_debug(vcpu))
        return 0;

And then in handle_exit:

    if (handled == 1 && kvm_arm_should_complete_emulated_instr_debug(vcpu))
        return 0;
    else
        return handled;

?
>
>>
>> > At least, I think it would be nice that the name reflect that this
>> > check is meant for emulated instructions.
>> >
>> > Otherwise:
>> >
>> > Reviewed-by: Julien Thierry <julien.thierry@....com>
>> >
>> > Thanks,
>>
>>
> Thanks,
> -Christoffer


--
Alex Bennée