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

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Message-ID: <20220719150422.GY1379820@ls.amr.corp.intel.com>
Date:   Tue, 19 Jul 2022 08:04:22 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v7 043/102] KVM: x86/mmu: Focibly use TDP MMU for TDX

On Mon, Jul 11, 2022 at 02:56:29PM +0000,
Sean Christopherson <seanjc@...gle.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@...el.com wrote:
> > From: Isaku Yamahata <isaku.yamahata@...el.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@...el.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@...il.com>