Re: [PATCH v13 17/22] x86/kexec: Flush cache of TDX private memory

[Dave Hansen]

On 9/18/23 05:08, Huang, Kai wrote:
> On Fri, 2023-09-15 at 10:50 -0700, Dave Hansen wrote:
>> On 9/15/23 10:43, Edgecombe, Rick P wrote:
>>> On Sat, 2023-08-26 at 00:14 +1200, Kai Huang wrote:
>>>> There are two problems in terms of using kexec() to boot to a new
>>>> kernel when the old kernel has enabled TDX: 1) Part of the memory
>>>> pages are still TDX private pages; 2) There might be dirty
>>>> cachelines associated with TDX private pages.
>>> Does TDX support hibernate?
>> No.
>>
>> There's a whole bunch of volatile state that's generated inside the CPU
>> and never leaves the CPU, like the ephemeral key that protects TDX
>> module memory.
>>
>> SGX, for instance, never even supported suspend, IIRC.  Enclaves just
>> die and have to be rebuilt.
> 
> Right.  AFAICT TDX cannot survive from S3 either.  All TDX keys get lost when
> system enters S3.  However I don't think TDX can be rebuilt after resume like
> SGX.  Let me confirm with TDX guys on this.

By "rebuilt" I mean all private data is totally destroyed and rebuilt
from scratch.  The SGX architecture provides zero help other than
delivering a fault and saying: "whoops all your data is gone".

> I think we can register syscore_ops->suspend for TDX, and refuse to suspend when
> TDX is enabled.  This covers hibernate case too.
> 
> In terms of how to check "TDX is enabled", ideally it's better to check whether
> TDX module is actually initialized, but the worst case is we can use
> platform_tdx_enabled(). (I need to think more on this)

*Ideally* the firmware would have a choke point where it could just tell
the OS that it can't suspend rather than the OS having to figure it out.