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

[Kuppuswamy, Sathyanarayanan]

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