[PATCH v4 1/3] arm64: kvm: support user space to detect RAS extension feature

[PATCH v4 1/3] arm64: kvm: support user space to detect RAS extension feature

[Dongjiu Geng]

Handle userspace's detection for RAS extension, because sometimes
the userspace needs to know the CPU's capacity

Signed-off-by: Dongjiu Geng <gengdongjiu@huawei.com>
---
 arch/arm64/kvm/reset.c   | 11 +++++++++++
 include/uapi/linux/kvm.h |  1 +
 2 files changed, 12 insertions(+)

diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
index d9e9697..1004039 100644
--- a/arch/arm64/kvm/reset.c
+++ b/arch/arm64/kvm/reset.c
@@ -64,6 +64,14 @@ static bool cpu_has_32bit_el1(void)
 	return !!(pfr0 & 0x20);
 }
 
+static bool kvm_arm_support_ras_extension(void)
+{
+	u64 pfr0;
+
+	pfr0 = read_system_reg(SYS_ID_AA64PFR0_EL1);
+	return !!(pfr0 & 0x10000000);
+}
+
 /**
  * kvm_arch_dev_ioctl_check_extension
  *
@@ -87,6 +95,9 @@ int kvm_arch_dev_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_ARM_PMU_V3:
 		r = kvm_arm_support_pmu_v3();
 		break;
+	case KVM_CAP_ARM_RAS_EXTENSION:
+		r = kvm_arm_support_ras_extension();
+		break;
 	case KVM_CAP_SET_GUEST_DEBUG:
 	case KVM_CAP_VCPU_ATTRIBUTES:
 		r = 1;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index f51d508..27fe556 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -883,6 +883,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_PPC_MMU_RADIX 134
 #define KVM_CAP_PPC_MMU_HASH_V3 135
 #define KVM_CAP_IMMEDIATE_EXIT 136
+#define KVM_CAP_ARM_RAS_EXTENSION 137
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
-- 
2.10.1

Re: [PATCH v4 1/3] arm64: kvm: support user space to detect RAS extension feature

[Christoffer Dall]

On Mon, Jun 26, 2017 at 08:45:43PM +0800, Dongjiu Geng wrote:
> Handle userspace's detection for RAS extension, because sometimes
> the userspace needs to know the CPU's capacity

Why?  Can you please provide some more rationale.

> 
> Signed-off-by: Dongjiu Geng <gengdongjiu@huawei.com>
> ---
>  arch/arm64/kvm/reset.c   | 11 +++++++++++
>  include/uapi/linux/kvm.h |  1 +
>  2 files changed, 12 insertions(+)
> 
> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index d9e9697..1004039 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -64,6 +64,14 @@ static bool cpu_has_32bit_el1(void)
>  	return !!(pfr0 & 0x20);
>  }
>  
> +static bool kvm_arm_support_ras_extension(void)
> +{
> +	u64 pfr0;
> +
> +	pfr0 = read_system_reg(SYS_ID_AA64PFR0_EL1);
> +	return !!(pfr0 & 0x10000000);
> +}

Why is this specific to KVM?  This seems to reveal information about the
underlying physical CPU, not specific to KVM at all, surely if userspace
is really supposed to be able to figure this out, it should not be KVM
specific.

Thanks,
-Christoffer

> +
>  /**
>   * kvm_arch_dev_ioctl_check_extension
>   *
> @@ -87,6 +95,9 @@ int kvm_arch_dev_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_ARM_PMU_V3:
>  		r = kvm_arm_support_pmu_v3();
>  		break;
> +	case KVM_CAP_ARM_RAS_EXTENSION:
> +		r = kvm_arm_support_ras_extension();
> +		break;
>  	case KVM_CAP_SET_GUEST_DEBUG:
>  	case KVM_CAP_VCPU_ATTRIBUTES:
>  		r = 1;
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index f51d508..27fe556 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -883,6 +883,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_PPC_MMU_RADIX 134
>  #define KVM_CAP_PPC_MMU_HASH_V3 135
>  #define KVM_CAP_IMMEDIATE_EXIT 136
> +#define KVM_CAP_ARM_RAS_EXTENSION 137
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
> -- 
> 2.10.1
> 

Re: [PATCH v4 1/3] arm64: kvm: support user space to detect RAS extension feature

[gengdongjiu]

Hi Christoffer,

On 2017/7/3 16:21, Christoffer Dall wrote:
> On Mon, Jun 26, 2017 at 08:45:43PM +0800, Dongjiu Geng wrote:
>> Handle userspace's detection for RAS extension, because sometimes
>> the userspace needs to know the CPU's capacity
> 
> Why?  Can you please provide some more rationale.

userspace mainly want to know whether CPU has RAS extension capability to decide whether need to specify the syndrome value.
if have, userspace specify the syndrome value. otherwise, not specify the value.

James ever suggest not want userspace to know the capability, and let KVM to judge the RAS extension capability.

but I consider it again, userspace know the RAS extension capability may be better, which can avoid KVM return error if
CPU does not support RAS extension.

could you give me some suggestion that whether let userspace to know the RAS extension capability?

> 
>>
>> Signed-off-by: Dongjiu Geng <gengdongjiu@huawei.com>
>> ---
>>  arch/arm64/kvm/reset.c   | 11 +++++++++++
>>  include/uapi/linux/kvm.h |  1 +
>>  2 files changed, 12 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
>> index d9e9697..1004039 100644
>> --- a/arch/arm64/kvm/reset.c
>> +++ b/arch/arm64/kvm/reset.c
>> @@ -64,6 +64,14 @@ static bool cpu_has_32bit_el1(void)
>>  	return !!(pfr0 & 0x20);
>>  }
>>  
>> +static bool kvm_arm_support_ras_extension(void)
>> +{
>> +	u64 pfr0;
>> +
>> +	pfr0 = read_system_reg(SYS_ID_AA64PFR0_EL1);
>> +	return !!(pfr0 & 0x10000000);
>> +}
> 
> Why is this specific to KVM?  This seems to reveal information about the
> underlying physical CPU, not specific to KVM at all, surely if userspace
> is really supposed to be able to figure this out, it should not be KVM
> specific.
  you are right. it should not be KVM specific, thanks for pointing it out.

> 
> Thanks,
> -Christoffer
> 
>> +
>>  /**
>>   * kvm_arch_dev_ioctl_check_extension
>>   *
>> @@ -87,6 +95,9 @@ int kvm_arch_dev_ioctl_check_extension(struct kvm *kvm, long ext)
>>  	case KVM_CAP_ARM_PMU_V3:
>>  		r = kvm_arm_support_pmu_v3();
>>  		break;
>> +	case KVM_CAP_ARM_RAS_EXTENSION:
>> +		r = kvm_arm_support_ras_extension();
>> +		break;
>>  	case KVM_CAP_SET_GUEST_DEBUG:
>>  	case KVM_CAP_VCPU_ATTRIBUTES:
>>  		r = 1;
>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> index f51d508..27fe556 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -883,6 +883,7 @@ struct kvm_ppc_resize_hpt {
>>  #define KVM_CAP_PPC_MMU_RADIX 134
>>  #define KVM_CAP_PPC_MMU_HASH_V3 135
>>  #define KVM_CAP_IMMEDIATE_EXIT 136
>> +#define KVM_CAP_ARM_RAS_EXTENSION 137
>>  
>>  #ifdef KVM_CAP_IRQ_ROUTING
>>  
>> -- 
>> 2.10.1
>>
> 
> .
> 

Re: [PATCH v4 1/3] arm64: kvm: support user space to detect RAS extension feature

[Christoffer Dall]

Hi Dongjiu,

On Tue, Jul 04, 2017 at 03:04:54PM +0800, gengdongjiu wrote:
> Hi Christoffer,
> 
> On 2017/7/3 16:21, Christoffer Dall wrote:
> > On Mon, Jun 26, 2017 at 08:45:43PM +0800, Dongjiu Geng wrote:
> >> Handle userspace's detection for RAS extension, because sometimes
> >> the userspace needs to know the CPU's capacity
> > 
> > Why?  Can you please provide some more rationale.
> 
> userspace mainly want to know whether CPU has RAS extension capability to decide whether need to specify the syndrome value.
> if have, userspace specify the syndrome value. otherwise, not specify the value.
> 
> James ever suggest not want userspace to know the capability, and let KVM to judge the RAS extension capability.
> 
> but I consider it again, userspace know the RAS extension capability may be better, which can avoid KVM return error if
> CPU does not support RAS extension.

So first of all, while I appreciate your efforts to explain your
rationale here, I am also asking you to provide a more comprehensive
commit message for the next version of the patch.

> 
> could you give me some suggestion that whether let userspace to know the RAS extension capability?
> 

I haven't thought much about it, but there are unanswered questions to
me.  For example, does the type and capabilities of the guest in any way
affect whether or not this features should be available?  Are there any
concerns with respect to migration?

And then the point I raised below, which is that this KVM-specific ioctl
is returning a hardware property of the CPU (the one the ioctl happens
to run on - what happens on a heterogeneous system?), but not really
what KVM can and cannot do.  At the very least this should be based on
a more generic in-kernel functionality, which reports the capabilities
of a system to KVM, and KVM could then tell userspace if it has a
certain capability.

Hope this helps,
-Christoffer