linux-kernel - Re: [RFC PATCH 20/21] KVM: x86: Force a prefetch fault's max mapping level to 4KB for TDX

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Message-ID: <1e6ced6acc5c733c88879a3febe09942d32175d6.camel@intel.com>
Date: Tue, 13 May 2025 23:20:18 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC: "Shutemov, Kirill" <kirill.shutemov@...el.com>, "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>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>, "tabba@...gle.com"
	<tabba@...gle.com>, "Li, Zhiquan1" <zhiquan1.li@...el.com>, "Du, Fan"
	<fan.du@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "michael.roth@....com"
	<michael.roth@....com>, "Weiny, Ira" <ira.weiny@...el.com>, "vbabka@...e.cz"
	<vbabka@...e.cz>, "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>,
	"ackerleytng@...gle.com" <ackerleytng@...gle.com>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>, "Peng, Chao P" <chao.p.peng@...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 20/21] KVM: x86: Force a prefetch fault's max mapping
 level to 4KB for TDX

On Thu, 2025-04-24 at 11:09 +0800, Yan Zhao wrote:
> Introduce a "prefetch" parameter to the private_max_mapping_level hook and
> enforce the max mapping level of a prefetch fault for private memory to be
> 4KB. This is a preparation to enable the ignoring huge page splitting in
> the fault path.
> 
> If a prefetch fault results in a 2MB huge leaf in the mirror page table,
> there may not be a vCPU available to accept the corresponding 2MB huge leaf
> in the S-EPT if the TD is not configured to receive #VE for page
> acceptance. 
> 

Can you elaborate on this case more. A vCPU may not be available? What does that
mean?

> Consequently, if a vCPU accepts the page at 4KB level, it will
> trigger an EPT violation to split the 2MB huge leaf generated by the
> prefetch fault.

The case is KVM_PRE_FAULT_MEMORY faults in 2MB, then guest accepts at 4k (which
it is not supposed to do)?

Then maybe the kvm_vm_dead() case I suggested in the other patch could handle
this case too, and this patch could be dropped?

> 
> Since handling the BUSY error from SEAMCALLs for huge page splitting is
> more comprehensive in the fault path, which is with kvm->mmu_lock held for
> reading, force the max mapping level of a prefetch fault of private memory
> to be 4KB to prevent potential splitting.