linux-kernel - Re: [PATCH v9 6/6] KVM: x86/mmu: Handle non-refcounted pages

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Date: Mon, 5 Feb 2024 19:23:24 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: David Stevens <stevensd@...omium.org>
Cc: Yu Zhang <yu.c.zhang@...ux.intel.com>, Isaku Yamahata <isaku.yamahata@...il.com>, 
	Zhi Wang <zhi.wang.linux@...il.com>, kvmarm@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v9 6/6] KVM: x86/mmu: Handle non-refcounted pages

On Mon, Sep 11, 2023, David Stevens wrote:
> From: David Stevens <stevensd@...omium.org>
> 
> Handle non-refcounted pages in __kvm_faultin_pfn. This allows the host
> to map memory into the guest that is backed by non-refcounted struct
> pages - for example, the tail pages of higher order non-compound pages
> allocated by the amdgpu driver via ttm_pool_alloc_page.
> 
> The bulk of this change is tracking the is_refcounted_page flag so that
> non-refcounted pages don't trigger page_count() == 0 warnings. This is
> done by storing the flag in an unused bit in the sptes. There are no
> bits available in PAE SPTEs, so non-refcounted pages can only be handled
> on TDP and x86-64.

Can you split this into two patches?  One to add all of the SPTE tracking, and
then one final patch to allow faulting in non-refcounted pages.  I want to isolate
the latter as much as possible, both for review purposes and in case something
goes awry and needs to be reverted.

> @@ -4254,13 +4265,18 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
>  static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  {
>  	struct kvm_memory_slot *slot = fault->slot;
> +	/*
> +	 * There are no extra bits for tracking non-refcounted pages in
> +	 * PAE SPTEs, so reject non-refcounted struct pages in that case.
> +	 */
> +	bool has_spte_refcount_bit = tdp_enabled && IS_ENABLED(CONFIG_X86_64);

Eh, just drop the local variable and do

		.allow_non_refcounted_struct_page = tdp_enabled &&
						    IS_ENABLED(CONFIG_X86_64);
(but keep the comment)