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

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Message-ID: <2b951e427c3f3f06fc310d151b7c9e960c32ec3f.camel@intel.com>
Date: Fri, 26 Sep 2025 16:02:19 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Hansen, Dave" <dave.hansen@...el.com>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>
CC: "Gao, Chao" <chao.gao@...el.com>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"Huang, Kai" <kai.huang@...el.com>, "kas@...nel.org" <kas@...nel.org>,
	"Annapurve, Vishal" <vannapurve@...gle.com>, "bp@...en8.de" <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v3 00/16] TDX: Enable Dynamic PAMT

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.