linux-kernel - Re: [RFC PATCH 14/21] KVM: x86/tdp_mmu: Invoke split_external_spt hook with exclusive mmu

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Message-ID: <724deab9c31b6e5cf30817b01446d344141178c8.camel@intel.com>
Date: Mon, 19 May 2025 20:21:13 +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 14/21] KVM: x86/tdp_mmu: Invoke split_external_spt
 hook with exclusive mmu_lock

On Mon, 2025-05-19 at 12:01 +0800, Yan Zhao wrote:
> On Sat, May 17, 2025 at 06:11:59AM +0800, Edgecombe, Rick P wrote:
> > On Fri, 2025-05-16 at 17:17 +0800, Yan Zhao wrote:
> > > > Shouldn't this BUG_ON be handled in the split_external_spt implementation? I
> > > > don't think we need another one.
> > > Ok. But kvm_x86_split_external_spt() is not for TDX only.
> > > Is it good for KVM MMU core to rely on each implementation to trigger BUG_ON?
> > 
> > It effectively is for TDX only. At least for the foreseeable future. The naming
> > basically means that people don't have to see "TDX" everywhere when they look in
> > the MMU code.
> Hmm, another reason to add the BUG_ON is to align it with remove_external_spte().
> There's also a KVM_BUG_ON() following the remove_external_spte hook.
> 
> I interpret this as error handling in the KVM MMU core, which returns "void",
> so issuing BUG_ON if ret != 0.

This is related to the other thread about how to handle demote failure. Let's
continue there.

But in general, the amount of KVM_BUG_ON()s we have for mirror EPT is a bit of a
code smell. It's not exclusive to this series. But I'd love if we could keep it
from getting worse.