Re: [PATCH v7 9/10] KVM: arm64: docs: document KVM support of pointer authentication

From: Amit Daniel Kachhap <amit.kachhap@arm.com>
To: Kristina Martsenko <kristina.martsenko@arm.com>,
	Julien Thierry <julien.thierry@arm.com>,
	linux-arm-kernel@lists.infradead.org
Cc: Christoffer Dall <christoffer.dall@arm.com>,
	Marc Zyngier <marc.zyngier@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Andrew Jones <drjones@redhat.com>,
	Dave Martin <Dave.Martin@arm.com>,
	Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>,
	kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org,
	Mark Rutland <mark.rutland@arm.com>,
	James Morse <james.morse@arm.com>
Subject: Re: [PATCH v7 9/10] KVM: arm64: docs: document KVM support of pointer authentication
Date: Thu, 21 Mar 2019 12:11:01 +0530	[thread overview]
Message-ID: <462c5f79-b0d6-8c4b-ce43-2dfcc0f3b7a6@arm.com> (raw)
In-Reply-To: <6d84723d-e00e-e89f-4c42-d48d0eb03443@arm.com>

Hi Julien/Kristina,

On 3/21/19 2:26 AM, Kristina Martsenko wrote:
> On 20/03/2019 18:06, Julien Thierry wrote:
>>
>>
>> On 20/03/2019 15:04, Kristina Martsenko wrote:
>>> On 20/03/2019 13:37, Julien Thierry wrote:
>>>> Hi Amit,
>>>>
>>>> On 19/03/2019 08:30, Amit Daniel Kachhap wrote:
>>>>> This adds sections for KVM API extension for pointer authentication.
>>>>> A brief description about usage of pointer authentication for KVM guests
>>>>> is added in the arm64 documentations.
>>>
>>> [...]
>>>
>>>>> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
>>>>> index 7de9eee..b5c66bc 100644
>>>>> --- a/Documentation/virtual/kvm/api.txt
>>>>> +++ b/Documentation/virtual/kvm/api.txt
>>>>> @@ -2659,6 +2659,12 @@ Possible features:
>>>>>   	  Depends on KVM_CAP_ARM_PSCI_0_2.
>>>>>   	- KVM_ARM_VCPU_PMU_V3: Emulate PMUv3 for the CPU.
>>>>>   	  Depends on KVM_CAP_ARM_PMU_V3.
>>>>> +	- KVM_ARM_VCPU_PTRAUTH_ADDRESS:
>>>>> +	- KVM_ARM_VCPU_PTRAUTH_GENERIC:
>>>>> +	  Enables Pointer authentication for the CPU.
>>>>> +	  Depends on KVM_CAP_ARM_PTRAUTH and only on arm64 architecture. If
>>>>> +	  set, then the KVM guest allows the execution of pointer authentication
>>>>> +	  instructions. Otherwise, KVM treats these instructions as undefined.
>>>>>   
>>>>
>>>> Overall I feel one could easily get confused to whether
>>>> PTRAUTH_ADDRESS/GENERIC are two individual features, whether one is a
>>>> superset of the other, if the names are just an alias of one another, etc...
>>>>
>>>> I think the doc should at least stress out that *both* flags are
>>>> required to enable ptrauth in a guest. However it raises the question,
>>>> if we don't plan to support the features individually (because we
>>>> can't), should we really expose two feature flags? I seems odd to
>>>> introduce two flags that only do something if used together...
>>>
>>> Why can't we support the features individually? For example, if we ever
>>> get a system where all CPUs support address authentication and none of
>>> them support generic authentication, then we could still support address
>>> authentication in the guest.
>>>
>>>
>>
>> That's a good point, I didn't think of that.
>>
>> Although, currently we don't have a way to detect that we are in such a
>> configuration. So as is, both flags are required to enable either
>> feature, and I feel the documentation should be clear on that aspect.
> 
> For now we only support enabling both features together, so both flags
> need to be set. I agree that the documentation should be made clear on this.
> 
> In the future, if we need to, we can add "negative" cpucaps to detect
> that a feature is absent on all CPUs.
> 
>>
>> Another option would be to introduce a flag that enables both for now,
>> and if one day we decide to support the configuration you mentioned we
>> could add "more modular" flags that allow you to control those features
>> individually. While a bit cumbersome, I would find that less awkward
>> than having two flags that only do something if both are present.
> 
> That would work too.
> 
> I find it more logical to have two flags since there are two features
> (two ID register fields), and KVM enables two features for the guest.
> The fact that KVM does not currently support enabling them separately is
> a KVM implementation choice, and could change in the future.
Kristina, this comments of yours is actually what this patch series is 
trying to do. I should have added more details about the necessity of 
keeping two flags and enhancement of them is actually a future work.

Thanks,
Amit Daniel
> 
> Thanks,
> Kristina
> 