linux-kernel - Re: [PATCH v10 005/108] KVM: TDX: Initialize the TDX module when loading the KVM intel kernel module

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Date:   Thu, 17 Nov 2022 09:33:03 -0800
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Erdem Aktas <erdemaktas@...gle.com>
Cc:     Isaku Yamahata <isaku.yamahata@...il.com>,
        "Huang, Kai" <kai.huang@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>
Subject: Re: [PATCH v10 005/108] KVM: TDX: Initialize the TDX module when
 loading the KVM intel kernel module

On Tue, Nov 15, 2022 at 04:22:13AM -0800,
Erdem Aktas <erdemaktas@...gle.com> wrote:

> On Mon, Nov 14, 2022 at 3:18 PM Isaku Yamahata <isaku.yamahata@...il.com> wrote:
> > > I think you should explain why MOVDIR64B is required, otherwise this just comes
> > > out of blue.
> > >
> > > Btw, is this absolutely required?  TDX also supports Li-mode, which doesn't have
> > > integrity check.  So theoretically with Li-mode, normal zeroing is also OK but
> > > doesn't need to use MOVDIR64B.
> > >
> > > That being said, do we have a way to tell whether TDX works in Ci or Li mode?
> >
> > As long as I don't know.  When clearing page, we can use
> > if (featuremovdir64b) movdir64b else memset(0).
> 
> -- sorry for resending the same email, for some reason my previous
> email was not in plain text mode--
> As far as I know, Li mode and Ci mode both are working very similarly
> and require movdir64b to clear any poison bit previously set. Why
> would movdir64b is not supported in a tdx capable cpu?

In practice, movdir64b can be safely assumed to be supported, I think.  Strictly
it's not guaranteed by SDM.
There are two options.

1. Assume modir64b. If it's not supported, KVM refuses to enable TDX.
   + Don't care CI-mode or Li-mode
   - not strictly conforming to SDM. In practice, it won't matter.
2. Don't assume movdir64b.  If movdir64b not supported, use memset(0).
   + strictly conforming to SDM
   - needs to care Ci-mode or Li-mode.

So I'll choose option 1.
-- 
Isaku Yamahata <isaku.yamahata@...il.com>