linux-kernel - Re: [RFC v1 09/26] x86/tdx: Handle CPUID via #VE

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Date:   Sun, 7 Feb 2021 14:45:26 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Andy Lutomirski <luto@...nel.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v1 09/26] x86/tdx: Handle CPUID via #VE


> On Feb 7, 2021, at 2:31 PM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 2/7/21 12:29 PM, Kirill A. Shutemov wrote:
>>> Couldn't you just have one big helper that takes *all* the registers
>>> that get used in any TDVMCALL and sets all the rcx bits?  The users
>>> could just pass 0's for the things they don't use.
>>> 
>>> Then you've got the ugly inline asm in one place.  It also makes it
>>> harder to screw up the 'rcx' mask and end up passing registers you
>>> didn't want into a malicious VMM.
>> For now we only pass down R10-R15, but the interface allows to pass down
>> much wider set of registers, including XMM. How far do we want to get it?
> 
> Just do what we immediately need: R10-R15
> .
> 

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.