Re: [PATCH 1/3] KVM: nVMX: Always flush vpid02 on first use

[Sean Christopherson]

On Wed, Jun 17, 2026, Kai Huang wrote:
> On Tue, 2026-06-16 at 21:46 +0000, Yosry Ahmed wrote:
> > Make sure vpid02 is always flushed on first use by setting last_vpid=0
> > when allocating vpid02.  nested_vmx_transition_tlb_flush() will always
> > detect a VPID change on first VM-Enter after VMXON, because VPID=0 in
> > vmcb12 is not allowed if L1 enables VPID.
> 
> vmcs12 :-)
> 
> > 
> > This avoids using stale TLB entries from a previous lifetime of the
> > VPID, that might have been associated with a different vCPU (or a
> > completely different VM).
> > 
> > Note that last_vpid is already being initialized as 0 when the vCPU is
> > created, but it is not reset when vpid02 is freed on VMXOFF. Hence, the
> > problem can only occur if L1 does VMXOFF -> VMXON, runs an L2, and KVM
> > happens to reuse a VPID that has TLB entries on the physical CPU.
> 
> Not sure whether it's better to set it to 0 in free_nested(), which also resets
> some other nested fields to clean slate AFAICT?

It needs to be set on first use, for the same reason that kvm_mmu_load() flushes
the root:

	/*
	 * Flush any TLB entries for the new root, the provenance of the root
	 * is unknown.  Even if KVM ensures there are no stale TLB entries
	 * for a freed root, in theory another hypervisor could have left
	 * stale entries.  Flushing on alloc also allows KVM to skip the TLB
	 * flush when freeing a root (see kvm_tdp_mmu_put_root()).
	 */
	kvm_x86_call(flush_tlb_current)(vcpu);