Messages in this thread | | | Subject | Re: [PATCH v7 9/10] KVM: arm64: docs: document KVM support of pointer authentication | From | Amit Daniel Kachhap <> | Date | Thu, 21 Mar 2019 12:11:01 +0530 |
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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 >
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