Re: [PATCH v3 00/16] TDX: Enable Dynamic PAMT

Edgecombe, Rick P
2025-09-26T16:03:24Z

On Fri, 2025-09-26 at 07:09 -0700, Dave Hansen wrote:
> On 9/25/25 19:28, Yan Zhao wrote:
> > > Lastly, Yan raised some last minute doubts internally about TDX
> > > module locking contention. I’m not sure there is a problem, but
> > > we
> > > can come to an agreement as part of the review.
> > Yes, I found a contention issue that prevents us from dropping the
> > global lock. I've also written a sample test that demonstrates this
> > contention.
> But what is the end result when this contention happens? Does
> everyone livelock?

You get a TDX_OPERAND_BUSY error code returned. Inside the TDX module
each lock is a try lock. The TDX module tries to take a sequence of
locks and if it meets any contention it will release them all and
return the TDX_OPERAND_BUSY. Some paths in KVM cannot handle failure
and so don't have a way to handle the error.

So another option to handling this is just to retry until you succeed.
Then you have a very strange spinlock with a heavyweight inner loop.
But since each time the locks are released, some astronomical bad
timing might prevent forward progress. On the KVM side we have avoided
looping. Although, I think we have not exhausted this path.