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SubjectRe: [PATCH v2 1/2] KVM: nVMX: don't use vcpu->arch.efer when checking host state on nested state load
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On 11/15/21 16:50, Sean Christopherson wrote:
> When loading nested state, don't use check vcpu->arch.efer to get the
> L1 host's 64-bit vs. 32-bit state and don't check it for consistency
> with respect to VM_EXIT_HOST_ADDR_SPACE_SIZE, as register state in vCPU
> may be stale when KVM_SET_NESTED_STATE is called and conceptually does
> not exist. When the CPU is in non-root mode, i.e. when restoring L2
> state in KVM, there is no snapshot of L1 host state, it is (conditionally)
> loaded on VM-Exit. E.g. EFER is either preserved on exit, loaded from the
> VMCS (vmcs12 in this case), or loaded from the MSR load list.
>
> Use vmcs12.VM_EXIT_HOST_ADDR_SPACE_SIZE to determine the target mode of
> the L1 host, as it is the source of truth in this case. Perform the EFER
> vs. vmcs12.VM_EXIT_HOST_ADDR_SPACE_SIZE consistency check only on VM-Enter,
> as conceptually there's no "current" L1 EFER to check.
>
> Note, KVM still checks vmcs12.HOST_EFER for consistency if
> if vmcs12.VM_EXIT_LOAD_IA32_EFER is set, i.e. this skips only the check
> against current vCPU state, which does not exist, when loading nested state.

Queued with some further edits and nested_vmx_check_address_state_size
renamed to nested_vmx_check_address_*space*_size.

I think the "!!" are best left in place though, because "!!(a & b)" is
idiomatic. Comparing "!(a & b)" would leave the reader wondering about
the inversion, and "(bool)(a & b)" is just too ugly and magic. The
compiler anyway converts the "!!" to "!= 0" very early on, and never
performs back-to-back logical NOTs.

Paolo

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