Re: [PATCH v3 2/3] KVM: VMX: Explicitly track TDX VMs' root level instead of guessing it from CPUID

[Xiaoyao Li]

On 9/3/2026 7:09 AM, Sean Christopherson wrote:
> Explicitly track the root level for TDX VMs instead of trying to infer the
> depth of the paging tree based on an individual vCPU's CPUID information.
> Applying KVM's existing logic to select the root level to TDX is flawed as
> nothing *requires* userspace to fill in the correct guest.MAXPHYADDR for a
> vCPU's CPUID.  Guessing at the correct root level is also ridiculous given
> that userspace has already told KVM the root level during TD initialization.
> 
> Relying on userspace to set the expected/correct CPUID lets a misbehaving
> userspace trip the KVM_BUG_ON() in tdx_load_mmu_pgd() by configuring guest
> CPUID to use an "incorrect" guest.MAXPHYADDR.
> 
> Don't use kvm_gfn_direct_bits() to infer the mirror root level, as the
> connection between TDX's one and only "direct" bit and the predetermined
> root level is a TDX implementation detail.  I.e. avoid baking in the
> assumption that there is exactly one "direct bits", that the one bit is a
> pivot between normal and mirror roots, and that the pivot bit is the most
> significant bit of the effective GPA space.  For the same reason, set the
> root level and direct bits in TDX code, i.e. don't provide a helper in the
> MMU, because from the MMU's perspective, they are two separate concepts.
> 
> Keep gfn_direct_bits even though it can be trivially derived from
> mirror_root_level as saving a whole eight bytes per VM is meaningless,
> keeping the TDX details buried in TDX would require a kvm_x86_ops hook, and
> the value is queried fairly often and in hot paths.
> 
> And for the moment, keep the S-bit sanity check in tdx_load_mmu_pgd(), even
> though it really only needs to ensure the incoming level matches the
> preconfigured mirror root level.  Because KVM manually configures the
> S-bit location, there's technically a risk that the S-bit location and
> mirror root level could get out of sync.  That can be addressed by more
> programmatically computing the S-bit, but that doesn't need to be done now.
> 
> Cc: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Cc: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> Cc: Kai Huang <kai.huang@xxxxxxxxx>
> Cc: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Fixes: 20d913729c11 ("KVM: x86/mmu: Taking guest pa into consideration when calculate tdp level")
> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Tested-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Tested-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
> Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>