Re: [RFC 07/19] KVM: x86/mmu: Factor wrprot for nested PML out of make_spte

[Sean Christopherson <seanjc@google.com>]

On Wed, Nov 10, 2021, Ben Gardon wrote:
> When running a nested VM, KVM write protects SPTEs in the EPT/NPT02
> instead of using PML for dirty tracking. This avoids expensive
> translation later, when emptying the Page Modification Log. In service
> of removing the vCPU pointer from make_spte, factor the check for nested
> PML out of the function.

Aha!  The dependency on @vcpu can be avoided without having to take a flag from
the caller.  The shadow page has everything we need.  The check is really "is this
a page for L2 EPT".  The kvm_x86_ops.cpu_dirty_log_size gets us the EPT part, and
kvm_mmu_page.guest_mode gets us the L2 part.

Compile tested only...

From 773414e4fd7010c38ac89221d16089f3dcc57467 Mon Sep 17 00:00:00 2001
From: Sean Christopherson <seanjc@google.com>
Date: Wed, 17 Nov 2021 18:08:42 -0800
Subject: [PATCH] KVM: x86/mmu: Use shadow page role to detect PML-unfriendly
 pages for L2

Rework make_spte() to query the shadow page's role, specifically whether
or not it's a guest_mode page, a.k.a. a page for L2, when determining if
the SPTE is compatible with PML.  This eliminates a dependency on @vcpu,
with a future goal of being able to create SPTEs without a specific vCPU.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/mmu_internal.h | 7 +++----
 arch/x86/kvm/mmu/spte.c         | 2 +-
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index 8ede43a826af..03882b2624c8 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -109,7 +109,7 @@ static inline int kvm_mmu_page_as_id(struct kvm_mmu_page *sp)
 	return kvm_mmu_role_as_id(sp->role);
 }

-static inline bool kvm_vcpu_ad_need_write_protect(struct kvm_vcpu *vcpu)
+static inline bool kvm_mmu_page_ad_need_write_protect(struct kvm_mmu_page *sp)
 {
 	/*
 	 * When using the EPT page-modification log, the GPAs in the CPU dirty
@@ -117,10 +117,9 @@ static inline bool kvm_vcpu_ad_need_write_protect(struct kvm_vcpu *vcpu)
 	 * on write protection to record dirty pages, which bypasses PML, since
 	 * writes now result in a vmexit.  Note, the check on CPU dirty logging
 	 * being enabled is mandatory as the bits used to denote WP-only SPTEs
-	 * are reserved for NPT w/ PAE (32-bit KVM).
+	 * are reserved for PAE paging (32-bit KVM).
 	 */
-	return vcpu->arch.mmu == &vcpu->arch.guest_mmu &&
-	       kvm_x86_ops.cpu_dirty_log_size;
+	return kvm_x86_ops.cpu_dirty_log_size && sp->role.guest_mode;
 }

 int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 0c76c45fdb68..84e64dbdd89e 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -101,7 +101,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,

 	if (sp->role.ad_disabled)
 		spte |= SPTE_TDP_AD_DISABLED_MASK;
-	else if (kvm_vcpu_ad_need_write_protect(vcpu))
+	else if (kvm_mmu_page_ad_need_write_protect(sp))
 		spte |= SPTE_TDP_AD_WRPROT_ONLY_MASK;

 	/*
--