Re: [PATCH v7 043/102] KVM: x86/mmu: Focibly use TDP MMU for TDX

Isaku Yamahata
2022-07-19T15:05:26Z

On Mon, Jul 11, 2022 at 02:56:29PM +0000,
Sean Christopherson <seanjc@google.com> wrote:

> s/Focibly/Forcibly, but that's a moot point because KVM shouldn't override the
> the module param. KVM should instead _require_ the TDP MMU to be enabled. E.g.
> if userspace disables the TDP MMU to workaround a fatal bug, then forcing the TDP
> MMU may silently expose KVM to said bug.
>
> And overriding tdp_enabled is just mind-boggling broken, all of the SPTE masks
> will be wrong.
>
> On Mon, Jun 27, 2022, isaku.yamahata@intel.com wrote:
> > From: Isaku Yamahata <isaku.yamahata@intel.com>
> >
> > In this patch series, TDX supports only TDP MMU and doesn't support legacy
> > MMU. Forcibly use TDP MMU for TDX irrelevant of kernel parameter to
> > disable TDP MMU.
>
> Do not refer to the "patch series", instead phrase the statement with respect to
> what KVM support.
>
> Require the TDP MMU for TDX guests, the so called "shadow" MMU does not
> support mapping guest private memory, i.e. does not support Secure-EPT.

Thanks for rewrite of the commit message. Now the TDP MMU is default, I'll change

> > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> > ---
> > arch/x86/kvm/mmu/tdp_mmu.c | 9 +++++++--
> > 1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 82f1bfac7ee6..7eb41b176d1e 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -18,8 +18,13 @@ int kvm_mmu_init_tdp_mmu(struct kvm *kvm)
> > {
> > struct workqueue_struct *wq;
> >
> > - if (!tdp_enabled || !READ_ONCE(tdp_mmu_enabled))
> > - return 0;
> > + /*
> > + * Because TDX supports only TDP MMU, forcibly use TDP MMU in the case
> > + * of TDX.
> > + */
> > + if (kvm->arch.vm_type != KVM_X86_TDX_VM &&
> > + (!tdp_enabled || !READ_ONCE(tdp_mmu_enabled)))
> > + return false;
>
> Yeah, no.
>
> if (!tdp_enabled || !READ_ONCE(tdp_mmu_enabled))
> return kvm->arch.vm_type == KVM_X86_TDX_VM ? -EINVAL : 0;

I'll use -EOPNOTSUPP instead of -EINVAL.
--
Isaku Yamahata <isaku.yamahata@gmail.com>