Re: [RFC PATCH v3 04/11] KVM: arm64: Setup a framework for hypercall bitmap firmware registers

[Jason Wang]

在 2022/1/5 上午3:49, Raghavendra Rao Ananta 写道:
> KVM regularly introduces new hypercall services to the guests without
> any consent from the Virtual Machine Manager (VMM). This means, the
> guests can observe hypercall services in and out as they migrate
> across various host kernel versions. This could be a major problem
> if the guest discovered a hypercall, started using it, and after
> getting migrated to an older kernel realizes that it's no longer
> available. Depending on how the guest handles the change, there's
> a potential chance that the guest would just panic.
>
> As a result, there's a need for the VMM to elect the services that
> it wishes the guest to discover. VMM can elect these services based
> on the kernels spread across its (migration) fleet. To remedy this,
> extend the existing firmware psuedo-registers, such as
> KVM_REG_ARM_PSCI_VERSION, for all the hypercall services available.



Haven't gone through the series but I wonder whether it's better to have 
a (e)BPF filter for this like seccomp.

Thanks


> These firmware registers are categorized based on the service call
> owners, and unlike the existing firmware psuedo-registers, they hold
> the features supported in the form of a bitmap.
>
> The capability, KVM_CAP_ARM_HVC_FW_REG_BMAP, is used to announce
> this extension, which returns the number of psuedo-firmware
> registers supported. During the VM initialization, the registers
> holds an upper-limit of the features supported by the corresponding
> registers. It's expected that the VMMs discover the features
> provided by each register via GET_ONE_REG, and writeback the
> desired values using SET_ONE_REG. KVM allows this modification
> only until the VM has started.
>
> Older VMMs can simply ignore the capability and the hypercall services
> will be exposed unconditionally to the guests, thus ensuring backward
> compatibility.
>
> In this patch, the framework adds the register only for ARM's standard
> secure services (owner value 4). Currently, this includes support only
> for ARM True Random Number Generator (TRNG) service, with bit-0 of the
> register representing mandatory features of v1.0. Other services are
> momentarily added in the upcoming patches.
>
> Signed-off-by: Raghavendra Rao Ananta<rananta@xxxxxxxxxx>