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Date:   Mon, 8 Feb 2021 09:47:27 -0800
From:   Sean Christopherson <seanjc@google.com>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andy Lutomirski <luto@kernel.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andi Kleen <ak@linux.intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Raj Ashok <ashok.raj@intel.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC v1 09/26] x86/tdx: Handle CPUID via #VE
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On Mon, Feb 08, 2021, Andy Lutomirski wrote:
> On Mon, Feb 8, 2021 at 9:11 AM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Sun, Feb 07, 2021, Andy Lutomirski wrote:
> > >
> 
> > > How much of the register state is revealed to the VMM when we do a TDVMCALL?
> > > Presumably we should fully sanitize all register state that shows up in
> > > cleartext on the other end, and we should treat all regs that can be modified
> > > by the VMM as clobbered.
> >
> > The guest gets to choose, with a few restrictions.  RSP cannot be exposed to the
> > host.  RAX, RCX, R10, and R11 are always exposed as they hold mandatory info
> > about the TDVMCALL (TDCALL fn, GPR mask, GHCI vs. vendor, and TDVMCALL fn).  All
> > other GPRs are exposed and clobbered if their bit in RCX is set, otherwise they
> > are saved/restored by the TDX-Module.
> >
> > I agree with Dave, pass everything required by the GHCI in the main routine, and
> > sanitize and save/restore all such GPRs.
> 
> Sounds okay to me.

One clarification: only non-volatile GPRs (R12-R15) need to be saved/restored.

And I think it makes sense to sanitize any exposed GPRs (that don't hold an
output value) after TDVMCALL to avoid speculating with a host-controlled value.