diff mbox series

[5.10,005/289] KVM: x86/mmu: Remove the defunct update_pte() paging hook

Message ID 20210517140305.340027792@linuxfoundation.org
State Superseded
Headers show
Series None | expand

Commit Message

Greg KH May 17, 2021, 1:58 p.m. UTC 
From: Sean Christopherson <seanjc@google.com>

commit c5e2184d1544f9e56140791eff1a351bea2e63b9 upstream.

Remove the update_pte() shadow paging logic, which was obsoleted by
commit 4731d4c7a077 ("KVM: MMU: out of sync shadow core"), but never
removed.  As pointed out by Yu, KVM never write protects leaf page
tables for the purposes of shadow paging, and instead marks their
associated shadow page as unsync so that the guest can write PTEs at
will.

The update_pte() path, which predates the unsync logic, optimizes COW
scenarios by refreshing leaf SPTEs when they are written, as opposed to
zapping the SPTE, restarting the guest, and installing the new SPTE on
the subsequent fault.  Since KVM no longer write-protects leaf page
tables, update_pte() is unreachable and can be dropped.

Reported-by: Yu Zhang <yu.c.zhang@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210115004051.4099250-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/x86/include/asm/kvm_host.h |    3 --
 arch/x86/kvm/mmu/mmu.c          |   49 +---------------------------------------
 arch/x86/kvm/x86.c              |    1 
 3 files changed, 2 insertions(+), 51 deletions(-)

Comments

Pavel Machek May 19, 2021, 7:56 p.m. UTC | #1 
Hi!

> From: Sean Christopherson <seanjc@google.com>

> 

> commit c5e2184d1544f9e56140791eff1a351bea2e63b9 upstream.

> 

> Remove the update_pte() shadow paging logic, which was obsoleted by

> commit 4731d4c7a077 ("KVM: MMU: out of sync shadow core"), but never

> removed.  As pointed out by Yu, KVM never write protects leaf page


First, this is cleanup, not a bugfix.

Second, AFAICT 4731d4c7a077 ("KVM: MMU: out of sync shadow core") is
not in 5.10, so this will break stuff according to the changelog.

Best regards,
								Pavel
								
-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Paolo Bonzini May 20, 2021, 9:57 a.m. UTC | #2 
On 19/05/21 21:56, Pavel Machek wrote:
> Hi!

> 

>> From: Sean Christopherson <seanjc@google.com>

>>

>> commit c5e2184d1544f9e56140791eff1a351bea2e63b9 upstream.

>>

>> Remove the update_pte() shadow paging logic, which was obsoleted by

>> commit 4731d4c7a077 ("KVM: MMU: out of sync shadow core"), but never

>> removed.  As pointed out by Yu, KVM never write protects leaf page

> 

> First, this is cleanup, not a bugfix.


It was reported to also fix a bug.  Commit 4731d4c7a077 is 2008 vintage, 
so it is certainly in 5.10.

Paolo

> Second, AFAICT 4731d4c7a077 ("KVM: MMU: out of sync shadow core") is

> not in 5.10, so this will break stuff according to the changelog.
diff mbox series

Patch

--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -358,8 +358,6 @@  struct kvm_mmu {
 	int (*sync_page)(struct kvm_vcpu *vcpu,
 			 struct kvm_mmu_page *sp);
 	void (*invlpg)(struct kvm_vcpu *vcpu, gva_t gva, hpa_t root_hpa);
-	void (*update_pte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
-			   u64 *spte, const void *pte);
 	hpa_t root_hpa;
 	gpa_t root_pgd;
 	union kvm_mmu_role mmu_role;
@@ -1019,7 +1017,6 @@  struct kvm_arch {
 struct kvm_vm_stat {
 	ulong mmu_shadow_zapped;
 	ulong mmu_pte_write;
-	ulong mmu_pte_updated;
 	ulong mmu_pde_zapped;
 	ulong mmu_flooded;
 	ulong mmu_recycled;
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -1715,13 +1715,6 @@  static int nonpaging_sync_page(struct kv
 	return 0;
 }
 
-static void nonpaging_update_pte(struct kvm_vcpu *vcpu,
-				 struct kvm_mmu_page *sp, u64 *spte,
-				 const void *pte)
-{
-	WARN_ON(1);
-}
-
 #define KVM_PAGE_ARRAY_NR 16
 
 struct kvm_mmu_pages {
@@ -3820,7 +3813,6 @@  static void nonpaging_init_context(struc
 	context->gva_to_gpa = nonpaging_gva_to_gpa;
 	context->sync_page = nonpaging_sync_page;
 	context->invlpg = NULL;
-	context->update_pte = nonpaging_update_pte;
 	context->root_level = 0;
 	context->shadow_root_level = PT32E_ROOT_LEVEL;
 	context->direct_map = true;
@@ -4402,7 +4394,6 @@  static void paging64_init_context_common
 	context->gva_to_gpa = paging64_gva_to_gpa;
 	context->sync_page = paging64_sync_page;
 	context->invlpg = paging64_invlpg;
-	context->update_pte = paging64_update_pte;
 	context->shadow_root_level = level;
 	context->direct_map = false;
 }
@@ -4431,7 +4422,6 @@  static void paging32_init_context(struct
 	context->gva_to_gpa = paging32_gva_to_gpa;
 	context->sync_page = paging32_sync_page;
 	context->invlpg = paging32_invlpg;
-	context->update_pte = paging32_update_pte;
 	context->shadow_root_level = PT32E_ROOT_LEVEL;
 	context->direct_map = false;
 }
@@ -4513,7 +4503,6 @@  static void init_kvm_tdp_mmu(struct kvm_
 	context->page_fault = kvm_tdp_page_fault;
 	context->sync_page = nonpaging_sync_page;
 	context->invlpg = NULL;
-	context->update_pte = nonpaging_update_pte;
 	context->shadow_root_level = kvm_mmu_get_tdp_level(vcpu);
 	context->direct_map = true;
 	context->get_guest_pgd = get_cr3;
@@ -4690,7 +4679,6 @@  void kvm_init_shadow_ept_mmu(struct kvm_
 	context->gva_to_gpa = ept_gva_to_gpa;
 	context->sync_page = ept_sync_page;
 	context->invlpg = ept_invlpg;
-	context->update_pte = ept_update_pte;
 	context->root_level = level;
 	context->direct_map = false;
 	context->mmu_role.as_u64 = new_role.as_u64;
@@ -4838,19 +4826,6 @@  void kvm_mmu_unload(struct kvm_vcpu *vcp
 }
 EXPORT_SYMBOL_GPL(kvm_mmu_unload);
 
-static void mmu_pte_write_new_pte(struct kvm_vcpu *vcpu,
-				  struct kvm_mmu_page *sp, u64 *spte,
-				  const void *new)
-{
-	if (sp->role.level != PG_LEVEL_4K) {
-		++vcpu->kvm->stat.mmu_pde_zapped;
-		return;
-        }
-
-	++vcpu->kvm->stat.mmu_pte_updated;
-	vcpu->arch.mmu->update_pte(vcpu, sp, spte, new);
-}
-
 static bool need_remote_flush(u64 old, u64 new)
 {
 	if (!is_shadow_present_pte(old))
@@ -4966,22 +4941,6 @@  static u64 *get_written_sptes(struct kvm
 	return spte;
 }
 
-/*
- * Ignore various flags when determining if a SPTE can be immediately
- * overwritten for the current MMU.
- *  - level: explicitly checked in mmu_pte_write_new_pte(), and will never
- *    match the current MMU role, as MMU's level tracks the root level.
- *  - access: updated based on the new guest PTE
- *  - quadrant: handled by get_written_sptes()
- *  - invalid: always false (loop only walks valid shadow pages)
- */
-static const union kvm_mmu_page_role role_ign = {
-	.level = 0xf,
-	.access = 0x7,
-	.quadrant = 0x3,
-	.invalid = 0x1,
-};
-
 static void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
 			      const u8 *new, int bytes,
 			      struct kvm_page_track_notifier_node *node)
@@ -5032,14 +4991,10 @@  static void kvm_mmu_pte_write(struct kvm
 
 		local_flush = true;
 		while (npte--) {
-			u32 base_role = vcpu->arch.mmu->mmu_role.base.word;
-
 			entry = *spte;
 			mmu_page_zap_pte(vcpu->kvm, sp, spte, NULL);
-			if (gentry &&
-			    !((sp->role.word ^ base_role) & ~role_ign.word) &&
-			    rmap_can_add(vcpu))
-				mmu_pte_write_new_pte(vcpu, sp, spte, &gentry);
+			if (gentry && sp->role.level != PG_LEVEL_4K)
+				++vcpu->kvm->stat.mmu_pde_zapped;
 			if (need_remote_flush(entry, *spte))
 				remote_flush = true;
 			++spte;
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -233,7 +233,6 @@  struct kvm_stats_debugfs_item debugfs_en
 	VCPU_STAT("halt_poll_fail_ns", halt_poll_fail_ns),
 	VM_STAT("mmu_shadow_zapped", mmu_shadow_zapped),
 	VM_STAT("mmu_pte_write", mmu_pte_write),
-	VM_STAT("mmu_pte_updated", mmu_pte_updated),
 	VM_STAT("mmu_pde_zapped", mmu_pde_zapped),
 	VM_STAT("mmu_flooded", mmu_flooded),
 	VM_STAT("mmu_recycled", mmu_recycled),