linux-kernel - Re: [RFC v2-fix-v4 1/1] x86/tdx: Skip WBINVD instruction for TDX guest

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Date:   Wed, 9 Jun 2021 10:28:14 -0700
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2-fix-v4 1/1] x86/tdx: Skip WBINVD instruction for TDX
 guest



On 6/9/21 9:12 AM, Andy Lutomirski wrote:
> On 6/9/21 8:09 AM, Dan Williams wrote:
>> On Tue, Jun 8, 2021 at 9:27 PM Andi Kleen <ak@...ux.intel.com> wrote:
>>>
>>>
>>> here is no resume path.
>>>
>>>> Host is free to go into S3 independent of any guest state.
>>>
>>> Actually my understanding is that none of the systems which support TDX
>>> support S3. S3 has been deprecated for a long time.
>>
>> Ok, I wanted to imply any power state that might power-off caches.
>>
>>>
>>>
>>>>    A hostile
>>>> host is free to do just enough cache management so that it can resume
>>>> from S3 while arranging for TDX guest dirty data to be lost. Does a
>>>> TDX guest go fatal if the cache loses power?
>>>
>>> That would be a machine check, and yes it would be fatal.
>>
>> Sounds good, so incorporating this and Andy's feedback:
>>
>> "TDX guests, like other typical guests, use standard ACPI mechanisms
>> to signal sleep state entry (including reboot) to the host. The ACPI
>> specification mandates WBINVD on any sleep state entry with the
>> expectation that the platform is only responsible for maintaining the
>> state of memory over sleep states, not preserving dirty data in any
>> CPU caches. ACPI cache flushing requirements pre-date the advent of
>> virtualization. Given guest sleep state entry does not affect any host
>> power rails it is not required to flush caches. The host is
>> responsible for maintaining cache state over its own bare metal sleep
>> state transitions that power-off the cache. A TDX guest, unlike a
>> typical guest, will machine check if the CPU cache is powered off."
>>
>> Andi, is that machine check behavior relative to power states
>> mentioned in the docs?
> 
> I don't think there's anything about power states.  There is a general
> documented mechanism to integrity-check TD guest memory, but it is *not*
> replay-resistant.  So, if the guest dirties a cache line, and the cache
> line is lost, it seems entirely plausible that the guest would get
> silently corrupted.
> 
> I would argue that, if this happens, it's a host, TD module, or
> architecture bug, and it's not the guest's fault.

If you want to apply this fix for all hypervisors (using boot_cpu_has
(X86_FEATURE_HYPERVISOR) check), then we don't need any TDX specific
reference in commit log right? It can be generalized for all VM guests.

agree?

> 
> --Andy
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer