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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Mon, 8 Feb 2021 09:35:03 -0800
Message-ID: <CALCETrWC8qEFuUBdknaU-u_BbB1+HEibAKSjygH_bX0fYR1hYg@mail.gmail.com>
Subject: Re: [RFC v1 09/26] x86/tdx: Handle CPUID via #VE
To:     Sean Christopherson <seanjc@google.com>
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>
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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.