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SubjectRe: [PATCH v9 4/5] KVM: arm64: Add capability to advertise ptrauth for guest
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Hi,

On 4/16/19 10:02 PM, Dave Martin wrote:
> On Fri, Apr 12, 2019 at 08:50:35AM +0530, Amit Daniel Kachhap wrote:
>> This patch advertises the capability of two cpu feature called address
>> pointer authentication and generic pointer authentication. These
>> capabilities depend upon system support for pointer authentication and
>> VHE mode.
>>
>> The current arm64 KVM partially implements pointer authentication and
>> support of address/generic authentication are tied together. However,
>> separate ABI requirements for both of them is added so that any future
>> isolated implementation will not require any ABI changes.
>>
>> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> Cc: Christoffer Dall <christoffer.dall@arm.com>
>> Cc: kvmarm@lists.cs.columbia.edu
>> ---
>> Changes since v8:
>> * Keep the capability check same for the 2 vcpu ptrauth features. [Dave Martin]
>>
>> Documentation/virtual/kvm/api.txt | 2 ++
>> arch/arm64/kvm/reset.c | 5 +++++
>> include/uapi/linux/kvm.h | 2 ++
>> 3 files changed, 9 insertions(+)
>>
>> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
>> index 9d202f4..56021d0 100644
>> --- a/Documentation/virtual/kvm/api.txt
>> +++ b/Documentation/virtual/kvm/api.txt
>> @@ -2756,9 +2756,11 @@ Possible features:
>> - KVM_ARM_VCPU_PTRAUTH_ADDRESS: Enables Address Pointer authentication
>> for the CPU and supported only on arm64 architecture.
>> Must be requested if KVM_ARM_VCPU_PTRAUTH_GENERIC is also requested.
>> + Depends on KVM_CAP_ARM_PTRAUTH_ADDRESS.
>
> What if KVM_CAP_ARM_PTRAUTH_ADDRESS is absent and
> KVM_ARM_VCPU_PTRAUTH_GENERIC is requested? By these rules, we have a
> contradiction: userspace both must request and must not request
> KVM_ARM_VCPU_PTRAUTH_ADDRESS.
>
> We could qualify as follows:
>
> Depends on KVM_CAP_ARM_PTRAUTH_ADDRESS.
> Must be requested if KVM_CAP_ARM_PTRAUTH_ADDRESS is present and
> KVM_ARM_VCPU_PTRAUTH_GENERIC is also requested.
ok agree. This makes it clear.
>
>> - KVM_ARM_VCPU_PTRAUTH_GENERIC: Enables Generic Pointer authentication
>> for the CPU and supported only on arm64 architecture.
>> Must be requested if KVM_ARM_VCPU_PTRAUTH_ADDRESS is also requested.
>> + Depends on KVM_CAP_ARM_PTRAUTH_GENERIC.
>
> Similarly.
>
> Or, we go back to having a single cap and a single feature, and add
> more caps/features later on if we decide it's possible to support
> address/generic auth separately later on.
>
> Otherwise, we end up with complex rules that can't be tested. This is a
> high price to pay for forwards compatibility: userspace's conformance to
> the rules can't be fully tested, so there's a fair chance it won't work
> properly anyway when hardware/KVM with just one auth type appears.
>
> [...]
>
> Thoughts?
I agree that single cpufeature/capability is a simple solution to
implement. The bifurcation of feature was done to reflect the different
ID register split up.

But the h/w implementation provides a same EL2 exception trap for both
the features and hence current implementation ties both of the features
together. I guess in future if this is limitation goes away then one
auth type is possible. Here I am not sure if the future h/w will retain
this merged exception trap and add 2 new separate exception trap in
addition to it.

I guess it will be probably simple split-up of this merged exception
trap. In this case there won't be any ABI change required as per current
implementation.

Thanks,
Amit Daniel


>
> Cheers
> ---Dave
>

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