linux-kernel - Re: [RFC PATCH v2 14/23] KVM: TDX: Split and inhibit huge mappings if a VMExit carries level info

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Message-ID: <aRvX9846Acx8NSZ8@yzhao56-desk.sh.intel.com>
Date: Tue, 18 Nov 2025 10:20:39 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>
CC: "Du, Fan" <fan.du@...el.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Hansen, Dave"
	<dave.hansen@...el.com>, "david@...hat.com" <david@...hat.com>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>, "tabba@...gle.com"
	<tabba@...gle.com>, "vbabka@...e.cz" <vbabka@...e.cz>, "michael.roth@....com"
	<michael.roth@....com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>, "ackerleytng@...gle.com"
	<ackerleytng@...gle.com>, "kas@...nel.org" <kas@...nel.org>, "Weiny, Ira"
	<ira.weiny@...el.com>, "Peng, Chao P" <chao.p.peng@...el.com>, "Yamahata,
 Isaku" <isaku.yamahata@...el.com>, "Annapurve, Vishal"
	<vannapurve@...gle.com>, "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"Miao, Jun" <jun.miao@...el.com>, "x86@...nel.org" <x86@...nel.org>,
	"pgonda@...gle.com" <pgonda@...gle.com>
Subject: Re: [RFC PATCH v2 14/23] KVM: TDX: Split and inhibit huge mappings
 if a VMExit carries level info

On Tue, Nov 18, 2025 at 09:04:20AM +0800, Huang, Kai wrote:
> On Fri, 2025-11-14 at 15:22 +0800, Yan Zhao wrote:
> > > Will 'level == PG_LEVEL_4K' in this case?  Or will this function return
> > > early right after check the eeq_type?
> > The function will return early right after check the eeq_type.
> 
> But for such case the fault handler will still return 2M and KVM will AUG 2M
> page?  Then if guest accepts 4K page, a new exit to KVM would happen?
>
> But this time KVM is able to find the info that guest is accepting 4K and KVM
> will split the 2M to 4K pages so we are good to go?

If guest accesses a private memory without first accepting it (like non-Linux
guests), the sequence is:
1. Guest accesses a private memory.
2. KVM finds it can map the GFN at 2MB. So, AUG 2MB pages.
3. Guest accepts the GFN at 4KB.
4. KVM receives a EPT violation with eeq_type of ACCEPT and level 4KB
5. KVM splits the 2MB mapping.
6. Guest accepts successfully and accesses the page.


If guest first accepts a private memory before accessing (like Linux guests),
the sequence is:
1. Guest accepts a private memory at 4KB.
2. KVM receives a EPT violation with eeq_type of ACCEPT and level 4KB.
3. KVM AUG 4KB.
4. Guest accepts successfully and accesses the page.