[RFC PATCH v2 04/25] KVM: x86/mmu: Support specifying a minimum TLB tag

[Yosry Ahmed]

In preparation for using the TLB tags allocator for SVM, which has a
range of ASIDs allocated for SEV/SNP, pass in a minimum TLB tag when
initializing the TLB tags allocator. The bitmap is conceptually shifted
such that bit=0 corresponds to tag=min.

Specifying the minimum value during initialization also makes the API
clearer, as the passed number of tags becomes the actual number of
*usable* tags, and tag=0 is explicitly excluded by the caller.

No functional change intended for VMX as VPID=0 is not used anyway.

Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
---
 arch/x86/kvm/mmu.h     |  2 +-
 arch/x86/kvm/mmu/mmu.c | 38 +++++++++++++++++++++++---------------
 arch/x86/kvm/vmx/vmx.h |  3 ++-
 3 files changed, 26 insertions(+), 17 deletions(-)

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 9a2916012cbff..cfffee92b8b71 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -337,7 +337,7 @@ static inline bool kvm_is_gfn_alias(struct kvm *kvm, gfn_t gfn)
 
 typedef unsigned int kvm_tlb_tag_t;
 
-int kvm_init_tlb_tags(unsigned int nr);
+int kvm_init_tlb_tags(kvm_tlb_tag_t min, unsigned int nr);
 void kvm_destroy_tlb_tags(void);
 kvm_tlb_tag_t kvm_alloc_tlb_tag(void);
 void kvm_free_tlb_tag(kvm_tlb_tag_t tag);
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index e021ed562502f..bf2e0c2205631 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -8197,24 +8197,26 @@ static struct {
 	spinlock_t	lock;
 	unsigned long	*bitmap;
 	unsigned int	nr;
+	kvm_tlb_tag_t	min;
 } tlb_tags;
 
-int kvm_init_tlb_tags(unsigned int nr)
+int kvm_init_tlb_tags(kvm_tlb_tag_t min, unsigned int nr)
 {
-	if (WARN_ON_ONCE(!nr))
-		return -EINVAL;
-
-	tlb_tags.bitmap = bitmap_zalloc(nr, GFP_KERNEL);
-	if (!tlb_tags.bitmap)
-		return -ENOMEM;
+	unsigned int end;
 
 	/*
 	 * 0 is the host's TLB tag for both VMX's VPID and SVM's ASID, and is
 	 * returned on failed allocations (e.g. no more tags left).
 	 */
-	__set_bit(0, tlb_tags.bitmap);
+	if (WARN_ON_ONCE(!min || !nr || check_add_overflow(min, nr, &end)))
+		return -EINVAL;
+
+	tlb_tags.bitmap = bitmap_zalloc(nr, GFP_KERNEL);
+	if (!tlb_tags.bitmap)
+		return -ENOMEM;
 
 	tlb_tags.nr = nr;
+	tlb_tags.min = min;
 	spin_lock_init(&tlb_tags.lock);
 	return 0;
 }
@@ -8229,30 +8231,36 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_destroy_tlb_tags);
 
 kvm_tlb_tag_t kvm_alloc_tlb_tag(void)
 {
-	kvm_tlb_tag_t tag;
+	unsigned int bit;
 
 	if (WARN_ON_ONCE(!tlb_tags.bitmap))
 		return 0;
 
 	guard(spinlock)(&tlb_tags.lock);
 
-	tag = find_first_zero_bit(tlb_tags.bitmap, tlb_tags.nr);
-	if (tag >= tlb_tags.nr)
+	bit = find_first_zero_bit(tlb_tags.bitmap, tlb_tags.nr);
+	if (bit >= tlb_tags.nr)
 		return 0;
 
-	__set_bit(tag, tlb_tags.bitmap);
-	return tag;
+	__set_bit(bit, tlb_tags.bitmap);
+	return tlb_tags.min + bit;
 }
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_alloc_tlb_tag);
 
 void kvm_free_tlb_tag(kvm_tlb_tag_t tag)
 {
-	if (!tag || WARN_ON_ONCE(tag >= tlb_tags.nr))
+	unsigned int bit;
+
+	if (!tag || WARN_ON_ONCE(tag < tlb_tags.min))
+		return;
+
+	bit = tag - tlb_tags.min;
+	if (WARN_ON_ONCE(bit >= tlb_tags.nr))
 		return;
 
 	guard(spinlock)(&tlb_tags.lock);
 
-	__clear_bit(tag, tlb_tags.bitmap);
+	__clear_bit(bit, tlb_tags.bitmap);
 }
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_free_tlb_tag);
 
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index d6d35637d94f8..0ddfe9626c126 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -336,7 +336,8 @@ static __always_inline u32 vmx_get_intr_info(struct kvm_vcpu *vcpu)
 
 static __always_inline int init_vpids(void)
 {
-	return enable_vpid ? kvm_init_tlb_tags(VMX_NR_VPIDS) : 0;
+	/* Exclude VPID=0 as it is used for the host */
+	return enable_vpid ? kvm_init_tlb_tags(1, VMX_NR_VPIDS - 1) : 0;
 }
 
 static __always_inline void destroy_vpids(void)
-- 
2.54.0.1136.gdb2ca164c4-goog