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

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Date:   Sat, 14 Oct 2017 07:18:50 -0700
From:   Christoffer Dall <cdall@...aro.org>
To:     Alex Bennée <alex.bennee@...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

On Fri, Oct 13, 2017 at 10:23:21AM +0100, Alex Bennée wrote:
> 
> 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 :-/
> 

Yeah, my suggestion was half-way a joke, but as Julien said, it should
reflect what it's trying to tell you as concretely as possible.

> > 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;

Ah yeah, my example was broken.

> 
> And then in handle_exit:
> 
>     if (handled == 1 && kvm_arm_should_complete_emulated_instr_debug(vcpu))
>         return 0;
>     else
>         return handled;
> 

Yes, looks good, assuming we can find a better name.

Thanks,
-Christoffer