Re: [PATCH v4 7/8] KVM: x86: nSVM: Handle restore of legacy nested state

[Yosry Ahmed]

On Thu, Feb 12, 2026 at 07:58:55AM -0800, Jim Mattson wrote:
> When nested NPT is enabled and KVM_SET_NESTED_STATE is used to restore an
> old checkpoint (without a valid gPAT), the current IA32_PAT value must be
> used as L2's gPAT.
> 
> Unfortunately, checkpoint restore is non-atomic, and the order in which
> state components are restored is not specified. Hence, the current IA32_PAT
> value may be restored by KVM_SET_MSRS after KVM_SET_NESTED_STATE.  To
> further complicate matters, there may be a KVM_GET_NESTED_STATE before the
> next KVM_RUN.
> 
> Introduce a new boolean, svm->nested.legacy_gpat_semantics. When set, hPAT
> updates are also applied to gPAT, preserving the old behavior (i.e. L2
> shares L1's PAT). Set this boolean when restoring legacy state (i.e. nested
> NPT is enabled, but no GPAT is provided) in KVM_SET_NESTED_STATE. Clear
> this boolean in svm_vcpu_pre_run(), to ensure that hPAT and gPAT are
> decoupled before the vCPU resumes execution.
> 
> Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>

Reviewed-by: Yosry Ahmed <yosry.ahmed@xxxxxxxxx>