On Thu, Oct 31, 2024 at 01:20:11PM -0700, Sean Christopherson wrote: >When getting the current VPID, e.g. to emulate a guest TLB flush, return >vpid01 if L2 is running but with VPID disabled, i.e. if VPID is disabled >in vmcs12. Architecturally, if VPID is disabled, then the guest and host >effectively share VPID=0. KVM emulates this behavior by using vpid01 when >running an L2 with VPID disabled (see prepare_vmcs02_early_rare()), and so >KVM must also treat vpid01 as the current VPID while L2 is active. > >Unconditionally treating vpid02 as the current VPID when L2 is active >causes KVM to flush TLB entries for vpid02 instead of vpid01, which >results in TLB entries from L1 being incorrectly preserved across nested >VM-Enter to L2 (L2=>L1 isn't problematic, because the TLB flush after >nested VM-Exit flushes vpid01). > >The bug manifests as failures in the vmx_apicv_test KVM-Unit-Test, as KVM >incorrectly retains TLB entries for the APIC-access page across a nested >VM-Enter. > >Opportunisticaly add comments at various touchpoints to explain the >architectural requirements, and also why KVM uses vpid01 instead of vpid02. > >All credit goes to Chao, who root caused the issue and identified the fix. > >Link: https://lore.kernel.org/all/ZwzczkIlYGX+QXJz@xxxxxxxxx >Fixes: 2b4a5a5d5688 ("KVM: nVMX: Flush current VPID (L1 vs. L2) for KVM_REQ_TLB_FLUSH_GUEST") >Cc: stable@xxxxxxxxxxxxxxx >Cc: Like Xu <like.xu.linux@xxxxxxxxx> >Debugged-by: Chao Gao <chao.gao@xxxxxxxxx> >Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx> I also ran the vmx_apicv_test KVM-Unit-Test. All failures are gone with this patch applied. So, Tested-by: Chao Gao <chao.gao@xxxxxxxxx>
