[v2,3/3] kvm: arm64: handle single-step of userspace mmio instructions

Message ID 20171109170021.2984-4-alex.bennee@linaro.org
State New
Headers show
Series
  • KVM: arm64: single step emulation instructions
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Commit Message

Alex Bennée Nov. 9, 2017, 5 p.m.
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.

The kvm_arm_handle_step_debug() helper tells us if we need to return
and sets up the exit_reason for us.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

---
v2
  - call helper directly from kvm_arch_vcpu_ioctl_run
---
 virt/kvm/arm/arm.c | 3 +++
 1 file changed, 3 insertions(+)

Comments

Julien Thierry Nov. 13, 2017, 11:04 a.m. | #1
Hi Alex,

On 09/11/17 17:00, 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.
> 
> The kvm_arm_handle_step_debug() helper tells us if we need to return
> and sets up the exit_reason for us.
> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> 
> ---
> v2
>    - call helper directly from kvm_arch_vcpu_ioctl_run
> ---
>   virt/kvm/arm/arm.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index 95cba0799828..2991adfaca9d 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -625,6 +625,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
>   		ret = kvm_handle_mmio_return(vcpu, vcpu->run);
>   		if (ret)
>   			return ret;
> +		if (kvm_arm_handle_step_debug(vcpu, vcpu->run))
> +			return 1;
> +

In the previous patch, kvm_arch_vcpu_ioctl_run return 0 when telling 
userspace about a debug exception. Shouldn't this branch return 0 
instead of 1?

Returning on non-zero for kvm_handle_mmio_return is done because it 
means there was an error. This is not the case for 
kvm_arm_handle_step_debug.

The description in the comment of kvm_arch_vcpu_ioctl_run is not very 
clear whether non-zero result should be used for errors or if only the 
negative values are treated as such, and positive values seems to be 
generally used to keep the vcpu going. So, I thought it might make sense 
to always return the same value upon debug exceptions.

Cheers,
Alex Bennée Nov. 13, 2017, 2:39 p.m. | #2
Julien Thierry <julien.thierry@arm.com> writes:

> Hi Alex,
>
> On 09/11/17 17:00, 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.
>>
>> The kvm_arm_handle_step_debug() helper tells us if we need to return
>> and sets up the exit_reason for us.
>>
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>>
>> ---
>> v2
>>    - call helper directly from kvm_arch_vcpu_ioctl_run
>> ---
>>   virt/kvm/arm/arm.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
>> index 95cba0799828..2991adfaca9d 100644
>> --- a/virt/kvm/arm/arm.c
>> +++ b/virt/kvm/arm/arm.c
>> @@ -625,6 +625,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>   		ret = kvm_handle_mmio_return(vcpu, vcpu->run);
>>   		if (ret)
>>   			return ret;
>> +		if (kvm_arm_handle_step_debug(vcpu, vcpu->run))
>> +			return 1;
>> +
>
> In the previous patch, kvm_arch_vcpu_ioctl_run return 0 when telling
> userspace about a debug exception. Shouldn't this branch return 0
> instead of 1?

Probably - although in practice it makes no difference. In QEMU for
example the test is if (run_ret < 0) to handle errors. Otherwise success
is assumed.

> Returning on non-zero for kvm_handle_mmio_return is done because it
> means there was an error. This is not the case for
> kvm_arm_handle_step_debug.
>
> The description in the comment of kvm_arch_vcpu_ioctl_run is not very
> clear whether non-zero result should be used for errors or if only the
> negative values are treated as such, and positive values seems to be
> generally used to keep the vcpu going. So, I thought it might make
> sense to always return the same value upon debug exceptions.

There is a subtle mis-match between what gets passed back to the ioctl
and what terminates the while() loop later on. As far as the ioctl is
concerned it's 0 success and - error. Once you get to the while loop
you'll only ever exit once ret is no longer > 0.

Anyway for consistency we should certainly return 0, I'll fix it on the
next iteration.

>
> Cheers,


--
Alex Bennée

Patch

diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
index 95cba0799828..2991adfaca9d 100644
--- a/virt/kvm/arm/arm.c
+++ b/virt/kvm/arm/arm.c
@@ -625,6 +625,9 @@  int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
 		ret = kvm_handle_mmio_return(vcpu, vcpu->run);
 		if (ret)
 			return ret;
+		if (kvm_arm_handle_step_debug(vcpu, vcpu->run))
+			return 1;
+
 	}
 
 	if (run->immediate_exit)