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SubjectRe: [PATCH v1 2/2] kvm: arm64: handle single-step of userspace mmio instructions
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On 06/10/17 14:47, Alex Bennée wrote:
>
> Marc Zyngier <marc.zyngier@arm.com> writes:
>
>> On 06/10/17 13:37, Marc Zyngier wrote:
>>> 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@linaro.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) {}
>>>>
>>>> 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);
>>>> int kvm_arm_vcpu_arch_set_attr(struct kvm_vcpu *vcpu,
>>>> struct kvm_device_attr *attr);
>>>> int kvm_arm_vcpu_arch_get_attr(struct kvm_vcpu *vcpu,
>>>> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
>>>> index dbadfaf850a7..a10a18c55c87 100644
>>>> --- a/arch/arm64/kvm/debug.c
>>>> +++ b/arch/arm64/kvm/debug.c
>>>> @@ -221,3 +221,24 @@ void kvm_arm_clear_debug(struct kvm_vcpu *vcpu)
>>>> }
>>>> }
>>>> }
>>>> +
>>>> +
>>>> +/*
>>>> + * When KVM has successfully emulated the instruction we might want to
>>>> + * return we a KVM_EXIT_DEBUG. We can only do this once the emulation
>>>> + * is complete though so for userspace emulations we have to wait
>>>> + * until we have re-entered KVM.
>>>> + *
>>>> + * Return > 0 to return to guest, 0 (and set exit_reason) on proper
>>>> + * exit to userspace.
>>>> + */
>>>> +
>>>> +int kvm_arm_maybe_return_debug(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>>> +{
>>>> + if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
>>>> + run->exit_reason = KVM_EXIT_DEBUG;
>>>> + run->debug.arch.hsr = ESR_ELx_EC_SOFTSTP_LOW << ESR_ELx_EC_SHIFT;
>>>> + return 0;
>>>> + }
>>>> + return 1;
>>>> +}
>>>> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
>>>> index c918d291cb58..7b04f59217bf 100644
>>>> --- a/arch/arm64/kvm/handle_exit.c
>>>> +++ b/arch/arm64/kvm/handle_exit.c
>>>> @@ -202,13 +202,10 @@ static int handle_trap_exceptions(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>>> 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;
>>>> - }
>>>> + if (handled)
>>>> + return kvm_arm_maybe_return_debug(vcpu, run);
>>>>
>>>> - return handled;
>>>> + return 0;
>>>> }
>>>>
>>>> /*
>>>> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
>>>> index b9f68e4add71..3d28fe2daa26 100644
>>>> --- a/virt/kvm/arm/arm.c
>>>> +++ b/virt/kvm/arm/arm.c
>>>> @@ -623,7 +623,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>>>
>>>> if (run->exit_reason == KVM_EXIT_MMIO) {
>>>> ret = kvm_handle_mmio_return(vcpu, vcpu->run);
>>>> - if (ret)
>>>> + if (ret < 1)
>>>> return ret;
>>>> }
>>>>
>>>> diff --git a/virt/kvm/arm/mmio.c b/virt/kvm/arm/mmio.c
>>>> index b6e715fd3c90..e43e3bd6222f 100644
>>>> --- a/virt/kvm/arm/mmio.c
>>>> +++ b/virt/kvm/arm/mmio.c
>>>> @@ -117,7 +117,8 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>>> vcpu_set_reg(vcpu, vcpu->arch.mmio_decode.rt, data);
>>>> }
>>>>
>>>> - return 0;
>>>> + /* If debugging in effect we may need to return now */
>>>> + return kvm_arm_maybe_return_debug(vcpu, run);
>>>
>>> Ah, that's how you do it. OK. Then the patch splitting is wrong, because
>>> everything is broken after patch #1.
>>
>> Actually, it is not broken at all. I'm just confused by the very
>> esoteric flow.
>
> We could just merge the whole patch in one but I wanted to show the
> difference between in-kernel and out-of-kernel emulation.
>
> I could also move the step handling to the mmio leg in
> kvm_arch_vcpu_ioctl_run but you mentioned you use the mmio completion
> elsewhere anyway?
Yes, look at the end of io_mem_abort(). This is used by the vgic to
complete a read emulation in the kernel.

And actually, this means that we shouldn't have to mess with
handle_exit. Just check for the return value of kvm_handle_mmio_return
in the call sites (including the one in io_mem_abort), and exit if we
need to single-step...

Thoughts?

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

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