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

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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>

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