linux-kernel - Re: [PATCH v10 7/8] KVM: x86/mmu: Track if sptes refer to refcounted pages

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Message-ID: <ZdjXuH4jBaAZCrgy@google.com>
Date: Fri, 23 Feb 2024 09:36:56 -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>, Maxim Levitsky <mlevitsk@...hat.com>, kvmarm@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v10 7/8] KVM: x86/mmu: Track if sptes refer to refcounted pages

On Wed, Feb 21, 2024, David Stevens wrote:
> + * Extra bits are only available for TDP SPTEs, since bits 62:52 are reserved
> + * for PAE paging, including NPT PAE. When a tracking bit isn't available, we
> + * will reject mapping non-refcounted struct pages.
> + */
> +static inline bool spte_has_refcount_bit(void)
> +{
> +	return tdp_enabled && IS_ENABLED(CONFIG_X86_64);
> +}
> +
> +static inline bool is_refcounted_page_spte(u64 spte)
> +{
> +	return !spte_has_refcount_bit() || (spte & SPTE_MMU_PAGE_REFCOUNTED);

The preferred way to handle this in KVM's MMU is to use a global variable, e.g.

  extern u64 __read_mostly shadow_refcounted_mask;

that avoids conditional branches when creating SPTEs, and arguably yields slightly
more readable code, e.g.

	return !shadow_refcounted_mask || (spte & shadow_refcounted_mask);