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

[Huang, Kai]

On Wed, 2026-06-17 at 06:03 -0700, Sean Christopherson wrote:
> 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);

I think you mean the "actual flush" needs to be done on the first use.  But
setting last_vpid to 0 is a setting which is to make sure the actual flush will
always be done on the first use, i.e., the actual flush will always be done on
the first use.  For this purpose seems to me there's no difference between
setting last_vpid to 0 in enter_vmx_operation() and free_nested(), but maybe I
am missing something.

But I guess doing it in enter_vmx_operation() matches the logic of "doing actual
flush on first use" more :-)