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: <b5af66343b3f5d4083ee875017c7449dea922006.camel@intel.com>
Date: Tue, 13 May 2025 23:06:48 +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 14/21] KVM: x86/tdp_mmu: Invoke split_external_spt
 hook with exclusive mmu_lock

On Thu, 2025-04-24 at 11:07 +0800, Yan Zhao wrote:
> +static int split_external_spt(struct kvm *kvm, gfn_t gfn, u64 old_spte,
> +			      u64 new_spte, int level)
> +{
> +	void *external_spt = get_external_spt(gfn, new_spte, level);
> +	int ret;
> +
> +	KVM_BUG_ON(!external_spt, kvm);
> +
> +	ret = static_call(kvm_x86_split_external_spt)(kvm, gfn, level, external_spt);
> +	KVM_BUG_ON(ret, kvm);

Shouldn't this BUG_ON be handled in the split_external_spt implementation? I
don't think we need another one.

> +
> +	return ret;
> +}
>  /**
>   * handle_removed_pt() - handle a page table removed from the TDP structure
>   *
> @@ -764,13 +778,13 @@ static u64 tdp_mmu_set_spte(struct kvm *kvm, int as_id, tdp_ptep_t sptep,
>  
>  	handle_changed_spte(kvm, as_id, gfn, old_spte, new_spte, level, false);
>  
> -	/*
> -	 * Users that do non-atomic setting of PTEs don't operate on mirror
> -	 * roots, so don't handle it and bug the VM if it's seen.
> -	 */
>  	if (is_mirror_sptep(sptep)) {
> -		KVM_BUG_ON(is_shadow_present_pte(new_spte), kvm);
> -		remove_external_spte(kvm, gfn, old_spte, level);
> +		if (!is_shadow_present_pte(new_spte))
> +			remove_external_spte(kvm, gfn, old_spte, level);
> +		else if (is_last_spte(old_spte, level) && !is_last_spte(new_spte, level))
> +			split_external_spt(kvm, gfn, old_spte, new_spte, level);
> +		else
> +			KVM_BUG_ON(1, kvm);

It might be worth a comment what this is looking for at this point. I think it's
that external EPT only support certain operations, so bug if any unsupported
operations are seen.

>  	}
>  
>  	return old_spte;