linux-kernel - Re: [RFC PATCH 12/21] KVM: TDX: Determine max mapping level according to vCPU's ACCEPT level

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Message-ID: <ad2c04d5d377257e8398775326a268e2aaaf807d.camel@intel.com>
Date: Mon, 19 May 2025 20:17:04 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC: "quic_eberman@...cinc.com" <quic_eberman@...cinc.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>, "Li,
 Zhiquan1" <zhiquan1.li@...el.com>, "thomas.lendacky@....com"
	<thomas.lendacky@....com>, "tabba@...gle.com" <tabba@...gle.com>, "Du, Fan"
	<fan.du@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"Weiny, Ira" <ira.weiny@...el.com>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"michael.roth@....com" <michael.roth@....com>, "vbabka@...e.cz"
	<vbabka@...e.cz>, "ackerleytng@...gle.com" <ackerleytng@...gle.com>, "Peng,
 Chao P" <chao.p.peng@...el.com>, "Shutemov, Kirill"
	<kirill.shutemov@...el.com>, "Annapurve, Vishal" <vannapurve@...gle.com>,
	"jroedel@...e.de" <jroedel@...e.de>, "Miao, Jun" <jun.miao@...el.com>,
	"pgonda@...gle.com" <pgonda@...gle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC PATCH 12/21] KVM: TDX: Determine max mapping level according
 to vCPU's ACCEPT level

On Mon, 2025-05-19 at 14:39 +0800, Yan Zhao wrote:
> > It sounds like Kirill is suggesting we do have to have demotion in the fault
> > path. IIRC it adds a lock, but the cost to skip fault path demotion seems to
> > be
> > adding up.
> Yes, though Kirill is suggesting to support demotion in the fault path, I
> still
> think that using tdx_gmem_private_max_mapping_level() might be more friendly
> to
> other potential scenarios, such as when the KVM core MMU requests TDX to
> perform
> page promotion, and TDX finds that promotion would consistently fail on a GFN.
> 
> Another important reason for not passing a max_fault_level into the fault
> struct
> is that the KVM MMU now has the hook private_max_mapping_level to determine a
> private fault's maximum level, which was introduced by commit f32fb32820b1
> ("KVM: x86: Add hook for determining max NPT mapping level"). We'd better not
> to
> introduce another mechanism if the same job can be accomplished via the
> private_max_mapping_level hook.

How about the alternative discussed on the thread with Kai? I don't think Kirill
was suggesting #VE based TDs need huge pages, just that they need to work with
4k accepts. Let's continue the discussion on that thread, because I think they
are all related. Once we conclude there we can iron out any remaining issues on
this specific patch.

> 
> The code in TDX huge page v8 [1][2] simply inherited the old implementation
> from
> its v1 [3], where the private_max_mapping_level hook had not yet been
> introduced
> for private faults.