[RFC PATCH v5 033/104] KVM: x86: Add infrastructure for stolen GPA bits

[isaku . yamahata]

From: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>

Add support in KVM's MMU for aliasing multiple GPAs (from a hardware
perspective) to a single GPA (from a memslot perspective). GPA aliasing
will be used to repurpose GPA bits as attribute bits, e.g. to expose an
execute-only permission bit to the guest. To keep the implementation
simple (relatively speaking), GPA aliasing is only supported via TDP.

Today KVM assumes two things that are broken by GPA aliasing.
  1. GPAs coming from hardware can be simply shifted to get the GFNs.
  2. GPA bits 51:MAXPHYADDR are reserved to zero.

With GPA aliasing, translating a GPA to GFN requires masking off the
repurposed bit, and a repurposed bit may reside in 51:MAXPHYADDR.

To support GPA aliasing, introduce the concept of per-VM GPA stolen bits,
that is, bits stolen from the GPA to act as new virtualized attribute
bits. A bit in the mask will cause the MMU code to create aliases of the
GPA. It can also be used to find the GFN out of a GPA coming from a tdp
fault.

To handle case (1) from above, retain any stolen bits when passing a GPA
in KVM's MMU code, but strip them when converting to a GFN so that the
GFN contains only the "real" GFN, i.e. never has repurposed bits set.

GFNs (without stolen bits) continue to be used to:
  - Specify physical memory by userspace via memslots
  - Map GPAs to TDP PTEs via RMAP
  - Specify dirty tracking and write protection
  - Look up MTRR types
  - Inject async page faults

Since there are now multiple aliases for the same aliased GPA, when
userspace memory backing the memslots is paged out, both aliases need to be
modified. Fortunately, this happens automatically. Since rmap supports
multiple mappings for the same GFN for PTE shadowing based paging, by
adding/removing each alias PTE with its GFN, kvm_handle_hva() based
operations will be applied to both aliases.

In the case of the rmap being removed in the future, the needed
information could be recovered by iterating over the stolen bits and
walking the TDP page tables.

For TLB flushes that are address based, make sure to flush both aliases
in the case of stolen bits.

Only support stolen bits in 64 bit guest paging modes (long, PAE).
Features that use this infrastructure should restrict the stolen bits to
exclude the other paging modes. Don't support stolen bits for shadow EPT.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
---
 arch/x86/include/asm/kvm_host.h |  2 ++
 arch/x86/kvm/mmu.h              | 51 +++++++++++++++++++++++++++++++++
 arch/x86/kvm/mmu/mmu.c          | 19 ++++++++++--
 arch/x86/kvm/mmu/paging_tmpl.h  | 25 +++++++++-------
 4 files changed, 84 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 208b29b0e637..d8b78d6abc10 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1235,7 +1235,9 @@ struct kvm_arch {
 	spinlock_t hv_root_tdp_lock;
 #endif
 
+#ifdef CONFIG_KVM_MMU_PRIVATE
 	gfn_t gfn_shared_mask;
+#endif
 };
 
 struct kvm_vm_stat {
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index e9fbb2c8bbe2..3fb530359f81 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -365,4 +365,55 @@ static inline gpa_t kvm_translate_gpa(struct kvm_vcpu *vcpu,
 		return gpa;
 	return translate_nested_gpa(vcpu, gpa, access, exception);
 }
+
+static inline gfn_t kvm_gfn_stolen_mask(struct kvm *kvm)
+{
+#ifdef CONFIG_KVM_MMU_PRIVATE
+	return kvm->arch.gfn_shared_mask;
+#else
+	return 0;
+#endif
+}
+
+static inline gpa_t kvm_gpa_stolen_mask(struct kvm *kvm)
+{
+	return gfn_to_gpa(kvm_gfn_stolen_mask(kvm));
+}
+
+static inline gpa_t kvm_gpa_unalias(struct kvm *kvm, gpa_t gpa)
+{
+	return gpa & ~kvm_gpa_stolen_mask(kvm);
+}
+
+static inline gfn_t kvm_gfn_unalias(struct kvm *kvm, gfn_t gfn)
+{
+	return gfn & ~kvm_gfn_stolen_mask(kvm);
+}
+
+static inline gfn_t kvm_gfn_shared(struct kvm *kvm, gfn_t gfn)
+{
+	return gfn | kvm_gfn_stolen_mask(kvm);
+}
+
+static inline gfn_t kvm_gfn_private(struct kvm *kvm, gfn_t gfn)
+{
+	return gfn & ~kvm_gfn_stolen_mask(kvm);
+}
+
+static inline gpa_t kvm_gpa_private(struct kvm *kvm, gpa_t gpa)
+{
+	return gpa & ~kvm_gpa_stolen_mask(kvm);
+}
+
+static inline bool kvm_is_private_gfn(struct kvm *kvm, gfn_t gfn)
+{
+	gfn_t mask = kvm_gfn_stolen_mask(kvm);
+
+	return mask && !(gfn & mask);
+}
+
+static inline bool kvm_is_private_gpa(struct kvm *kvm, gpa_t gpa)
+{
+	return kvm_is_private_gfn(kvm, gpa_to_gfn(gpa));
+}
 #endif
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 8e24f73bf60b..b68191aa39bf 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -276,11 +276,24 @@ static inline bool kvm_available_flush_tlb_with_range(void)
 static void kvm_flush_remote_tlbs_with_range(struct kvm *kvm,
 		struct kvm_tlb_range *range)
 {
-	int ret = -ENOTSUPP;
+	int ret = -EOPNOTSUPP;
+	u64 gfn_stolen_mask;
 
-	if (range && kvm_x86_ops.tlb_remote_flush_with_range)
+	/*
+	 * Fall back to the big hammer flush if there is more than one
+	 * GPA alias that needs to be flushed.
+	 */
+	gfn_stolen_mask = kvm_gfn_stolen_mask(kvm);
+	if (hweight64(gfn_stolen_mask) > 1)
+		goto generic_flush;
+
+	if (range && kvm_available_flush_tlb_with_range()) {
+		/* Callback should flush both private GFN and shared GFN. */
+		range->start_gfn = kvm_gfn_unalias(kvm, range->start_gfn);
 		ret = static_call(kvm_x86_tlb_remote_flush_with_range)(kvm, range);
+	}
 
+generic_flush:
 	if (ret)
 		kvm_flush_remote_tlbs(kvm);
 }
@@ -4010,7 +4023,7 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 	unsigned long mmu_seq;
 	int r;
 
-	fault->gfn = fault->addr >> PAGE_SHIFT;
+	fault->gfn = kvm_gfn_unalias(vcpu->kvm, gpa_to_gfn(fault->addr));
 	fault->slot = kvm_vcpu_gfn_to_memslot(vcpu, fault->gfn);
 
 	if (page_fault_handle_page_track(vcpu, fault))
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 5b5bdac97c7b..70aec31dee06 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -25,7 +25,8 @@
 	#define guest_walker guest_walker64
 	#define FNAME(name) paging##64_##name
 	#define PT_BASE_ADDR_MASK GUEST_PT64_BASE_ADDR_MASK
-	#define PT_LVL_ADDR_MASK(lvl) PT64_LVL_ADDR_MASK(lvl)
+	#define PT_LVL_ADDR_MASK(vcpu, lvl) (~kvm_gpa_stolen_mask(vcpu->kvm) & \
+					     PT64_LVL_ADDR_MASK(lvl))
 	#define PT_LVL_OFFSET_MASK(lvl) PT64_LVL_OFFSET_MASK(lvl)
 	#define PT_INDEX(addr, level) PT64_INDEX(addr, level)
 	#define PT_LEVEL_BITS PT64_LEVEL_BITS
@@ -44,7 +45,7 @@
 	#define guest_walker guest_walker32
 	#define FNAME(name) paging##32_##name
 	#define PT_BASE_ADDR_MASK PT32_BASE_ADDR_MASK
-	#define PT_LVL_ADDR_MASK(lvl) PT32_LVL_ADDR_MASK(lvl)
+	#define PT_LVL_ADDR_MASK(vcpu, lvl) PT32_LVL_ADDR_MASK(lvl)
 	#define PT_LVL_OFFSET_MASK(lvl) PT32_LVL_OFFSET_MASK(lvl)
 	#define PT_INDEX(addr, level) PT32_INDEX(addr, level)
 	#define PT_LEVEL_BITS PT32_LEVEL_BITS
@@ -58,7 +59,7 @@
 	#define guest_walker guest_walkerEPT
 	#define FNAME(name) ept_##name
 	#define PT_BASE_ADDR_MASK GUEST_PT64_BASE_ADDR_MASK
-	#define PT_LVL_ADDR_MASK(lvl) PT64_LVL_ADDR_MASK(lvl)
+	#define PT_LVL_ADDR_MASK(vcpu, lvl) PT64_LVL_ADDR_MASK(lvl)
 	#define PT_LVL_OFFSET_MASK(lvl) PT64_LVL_OFFSET_MASK(lvl)
 	#define PT_INDEX(addr, level) PT64_INDEX(addr, level)
 	#define PT_LEVEL_BITS PT64_LEVEL_BITS
@@ -75,7 +76,7 @@
 #define PT_GUEST_ACCESSED_MASK (1 << PT_GUEST_ACCESSED_SHIFT)
 
 #define gpte_to_gfn_lvl FNAME(gpte_to_gfn_lvl)
-#define gpte_to_gfn(pte) gpte_to_gfn_lvl((pte), PG_LEVEL_4K)
+#define gpte_to_gfn(vcpu, pte) gpte_to_gfn_lvl(vcpu, pte, PG_LEVEL_4K)
 
 /*
  * The guest_walker structure emulates the behavior of the hardware page
@@ -96,9 +97,9 @@ struct guest_walker {
 	struct x86_exception fault;
 };
 
-static gfn_t gpte_to_gfn_lvl(pt_element_t gpte, int lvl)
+static gfn_t gpte_to_gfn_lvl(struct kvm_vcpu *vcpu, pt_element_t gpte, int lvl)
 {
-	return (gpte & PT_LVL_ADDR_MASK(lvl)) >> PAGE_SHIFT;
+	return (gpte & PT_LVL_ADDR_MASK(vcpu, lvl)) >> PAGE_SHIFT;
 }
 
 static inline void FNAME(protect_clean_gpte)(struct kvm_mmu *mmu, unsigned *access,
@@ -395,7 +396,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
 		--walker->level;
 
 		index = PT_INDEX(addr, walker->level);
-		table_gfn = gpte_to_gfn(pte);
+		table_gfn = gpte_to_gfn(vcpu, pte);
 		offset    = index * sizeof(pt_element_t);
 		pte_gpa   = gfn_to_gpa(table_gfn) + offset;
 
@@ -460,7 +461,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
 	if (unlikely(errcode))
 		goto error;
 
-	gfn = gpte_to_gfn_lvl(pte, walker->level);
+	gfn = gpte_to_gfn_lvl(vcpu, pte, walker->level);
 	gfn += (addr & PT_LVL_OFFSET_MASK(walker->level)) >> PAGE_SHIFT;
 
 	if (PTTYPE == 32 && walker->level > PG_LEVEL_4K && is_cpuid_PSE36())
@@ -555,12 +556,14 @@ FNAME(prefetch_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 	gfn_t gfn;
 	kvm_pfn_t pfn;
 
+	WARN_ON(gpte & kvm_gpa_stolen_mask(vcpu->kvm));
+
 	if (FNAME(prefetch_invalid_gpte)(vcpu, sp, spte, gpte))
 		return false;
 
 	pgprintk("%s: gpte %llx spte %p\n", __func__, (u64)gpte, spte);
 
-	gfn = gpte_to_gfn(gpte);
+	gfn = gpte_to_gfn(vcpu, gpte);
 	pte_access = sp->role.access & FNAME(gpte_access)(gpte);
 	FNAME(protect_clean_gpte)(vcpu->arch.mmu, &pte_access, gpte);
 
@@ -656,6 +659,8 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 	WARN_ON_ONCE(gw->gfn != base_gfn);
 	direct_access = gw->pte_access;
 
+	WARN_ON(fault->addr & kvm_gpa_stolen_mask(vcpu->kvm));
+
 	top_level = vcpu->arch.mmu->root_level;
 	if (top_level == PT32E_ROOT_LEVEL)
 		top_level = PT32_ROOT_LEVEL;
@@ -1080,7 +1085,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 			continue;
 		}
 
-		gfn = gpte_to_gfn(gpte);
+		gfn = gpte_to_gfn(vcpu, gpte);
 		pte_access = sp->role.access;
 		pte_access &= FNAME(gpte_access)(gpte);
 		FNAME(protect_clean_gpte)(vcpu->arch.mmu, &pte_access, gpte);
-- 
2.25.1