linux-kernel - Re: [RFC PATCH 09/21] KVM: TDX: Enable 2MB mapping size after TD is RUNNABLE

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Message-ID: <aGSoDnODoG2/pbYn@yzhao56-desk.sh.intel.com>
Date: Wed, 2 Jul 2025 11:31:26 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC: "quic_eberman@...cinc.com" <quic_eberman@...cinc.com>, "Li, Xiaoyao"
	<xiaoyao.li@...el.com>, "Huang, Kai" <kai.huang@...el.com>, "Du, Fan"
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	<ackerleytng@...gle.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
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	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC PATCH 09/21] KVM: TDX: Enable 2MB mapping size after TD is
 RUNNABLE

On Wed, Jul 02, 2025 at 08:18:48AM +0800, Edgecombe, Rick P wrote:
> > > We need mmu write lock for demote, but as long as the order is:
> > > 1. set lpage_info
> > > 2. demote if needed
> > > 3. go to fault handler
> > > 
> > > Then (3) should have what it needs even if another fault races (1).
For now I implemented the sequence as

1. check lpage_info, if 2MB is already disabled for a GFN, goto 3.
2. if 2MB is not disabled,
   2.1 acquire write mmu_lock
   2.2 split the GFN mapping and kvm_flush_remote_tlbs() if split is performed
   2.3 update lpage_info to disable 2MB for the GFN
   2.4 release write mmu_lock
3. fault handler for the GFN

Note: write mmu_lock is held during 2.2 successfully splitting a huge GFN
entry and 2.3. So, it can guarantee that there's no 2MB mapping for the GFN
after 2.3.

Step 1 can help reduce the count of write mmu_lock from 17626 to 11.