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

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Date:   Tue, 8 Jun 2021 20:56:27 -0700
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.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 6/8/21 8:40 PM, Dan Williams wrote:
> ..."KVM gets away with it" is not a justification that TDX can stand
> on otherwise we would not be here fixing up ACPICA properly.
> 
> How about:
> 
> "TDX 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 TDX
> 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. If the host fails to manage caches over its sleep
> state transitions the guest..."

> 
> I don't know how to finish the last sentence. What does TDX do if it
> is resumed after host suspend and the host somehow arranged for dirty
> TDX lines to be lost.

TDX guest does not support S3. It will be disabled in ACPI tables. It
is a TDX firmware spec requirement. Please check the following spec,
sec 2.1

https://software.intel.com/content/dam/develop/external/us/en/documents/tdx-virtual-firmware-design-guide-rev-1.pdf

In TDX guest, we encounter cache flushes only in shutdown and reboot path.
So there is no resume path.


  Will that be noticed by TDX integrity
> mechanisms? I did not immediately find an answer to this with a brief
> look at the specs.

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer