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

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Message-ID: <ZuaurXwXUyEjP9WJ@yzhao56-desk.sh.intel.com>
Date: Sun, 15 Sep 2024 17:53:49 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>, Rick P Edgecombe
	<rick.p.edgecombe@...el.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	Yuan Yao <yuan.yao@...el.com>, Kai Huang <kai.huang@...el.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "dmatlack@...gle.com"
	<dmatlack@...gle.com>, "nik.borisov@...e.com" <nik.borisov@...e.com>
Subject: Re: [PATCH 09/21] KVM: TDX: Retry seamcall when TDX_OPERAND_BUSY
 with operand SEPT

On Sat, Sep 14, 2024 at 05:27:32PM +0800, Yan Zhao wrote:
> > Similarly, can tdh_mem_page_aug() actually contend with tdg_mem_page_accept()?
> > The page isn't yet mapped, so why would the guest be allowed to take a lock on
> > the S-EPT entry?
> Before tdg_mem_page_accept() accepts a gpa and set rwx bits in a SPTE, if second
> tdh_mem_page_aug() is called on the same gpa, the second one may contend with
> tdg_mem_page_accept().
> 
> But given KVM does not allow the second tdh_mem_page_aug(), looks the contention
> between tdh_mem_page_aug() and tdg_mem_page_accept() will not happen.
I withdraw the reply above.

tdh_mem_page_aug() and tdg_mem_page_accept() both attempt to modify the same
SEPT entry, leading to contention.
- tdg_mem_page_accept() first walks the SEPT tree with no lock to get the SEPT
  entry. It then acquire the guest side lock of the found SEPT entry before
  checking entry state.
- tdh_mem_page_aug() first walks the SEPT tree with shared lock to locate the
  SEPT entry to modify, it then aquires host side lock of the SEPT entry before
  checking entry state.