linux-kernel - Re: [PATCH 09/21] KVM: TDX: Retry seamcall when TDX_OPERAND

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Message-ID: <08533ab54cb482472176a057b8a10444ca32d10f.camel@intel.com>
Date: Mon, 14 Oct 2024 10:54:43 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>, "Edgecombe, Rick P"
	<rick.p.edgecombe@...el.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Yao, Yuan"
	<yuan.yao@...el.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"nik.borisov@...e.com" <nik.borisov@...e.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>
Subject: Re: [PATCH 09/21] KVM: TDX: Retry seamcall when TDX_OPERAND_BUSY with
 operand SEPT

On Thu, 2024-10-10 at 21:53 +0000, Edgecombe, Rick P wrote:
> On Thu, 2024-10-10 at 10:33 -0700, Sean Christopherson wrote:
> > > 
> > > 1st: "fault->is_private != kvm_mem_is_private(kvm, fault->gfn)" is found.
> > > 2nd-6th: try_cmpxchg64() fails on each level SPTEs (5 levels in total)
> 
> Isn't there a more general scenario:
> 
> vcpu0                              vcpu1
> 1. Freezes PTE
> 2. External op to do the SEAMCALL
> 3.                                 Faults same PTE, hits frozen PTE
> 4.                                 Retries N times, triggers zero-step
> 5. Finally finishes external op
> 
> Am I missing something?

I must be missing something.  I thought KVM is going to retry internally for
step 4 (retries N times) because it sees the frozen PTE, but will never go back
to guest after the fault is resolved?  How can step 4 triggers zero-step?