LoongArch: KVM: Fix VM migration failure with PTW enabled

With PTW disabled system, bit _PAGE_DIRTY is a HW bit for page writing.
However with PTW enabled system, bit _PAGE_WRITE is also a "HW bit" for
page writing, because hardware synchronizes _PAGE_WRITE to _PAGE_DIRTY
automatically. Previously, _PAGE_WRITE is treated as a SW bit to record
the page writeable attribute for the fast page fault handling in the
secondary MMU, however with PTW enabled machine, this bit is used by HW
already (so setting it will silence the TLB modify exception).

Here define KVM_PAGE_WRITEABLE with the SW bit _PAGE_MODIFIED, so that
it can work on both PTW disabled and enabled machines. And for HW write
bits, both _PAGE_DIRTY and _PAGE_WRITE are set or clear together.

Cc: stable@vger.kernel.org
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Author: bibo-mao

arch/loongarch/include/asm/kvm_mmu.h

@@ -16,6 +16,13 @@
  */
 #define KVM_MMU_CACHE_MIN_PAGES	(CONFIG_PGTABLE_LEVELS - 1)
 
+/*
+ * _PAGE_MODIFIED is a SW pte bit, it records page ever written on host
+ * kernel, on secondary MMU it records the page writeable attribute, in
+ * order for fast path handling.
+ */
+#define KVM_PAGE_WRITEABLE	_PAGE_MODIFIED
+
 #define _KVM_FLUSH_PGTABLE	0x1
 #define _KVM_HAS_PGMASK		0x2
 #define kvm_pfn_pte(pfn, prot)	(((pfn) << PFN_PTE_SHIFT) | pgprot_val(prot))
@@ -52,10 +59,10 @@ static inline void kvm_set_pte(kvm_pte_t *ptep, kvm_pte_t val)
 	WRITE_ONCE(*ptep, val);
 }
 
-static inline int kvm_pte_write(kvm_pte_t pte) { return pte & _PAGE_WRITE; }
-static inline int kvm_pte_dirty(kvm_pte_t pte) { return pte & _PAGE_DIRTY; }
 static inline int kvm_pte_young(kvm_pte_t pte) { return pte & _PAGE_ACCESSED; }
 static inline int kvm_pte_huge(kvm_pte_t pte) { return pte & _PAGE_HUGE; }
+static inline int kvm_pte_dirty(kvm_pte_t pte) { return pte & __WRITEABLE; }
+static inline int kvm_pte_writeable(kvm_pte_t pte) { return pte & KVM_PAGE_WRITEABLE; }
 
 static inline kvm_pte_t kvm_pte_mkyoung(kvm_pte_t pte)
 {
@@ -69,12 +76,12 @@ static inline kvm_pte_t kvm_pte_mkold(kvm_pte_t pte)
 
 static inline kvm_pte_t kvm_pte_mkdirty(kvm_pte_t pte)
 {
-	return pte | _PAGE_DIRTY;
+	return pte | __WRITEABLE;
 }
 
 static inline kvm_pte_t kvm_pte_mkclean(kvm_pte_t pte)
 {
-	return pte & ~_PAGE_DIRTY;
+	return pte & ~__WRITEABLE;
 }
 
 static inline kvm_pte_t kvm_pte_mkhuge(kvm_pte_t pte)
@@ -87,6 +94,11 @@ static inline kvm_pte_t kvm_pte_mksmall(kvm_pte_t pte)
 	return pte & ~_PAGE_HUGE;
 }
 
+static inline kvm_pte_t kvm_pte_mkwriteable(kvm_pte_t pte)
+{
+	return pte | KVM_PAGE_WRITEABLE;
+}
+
 static inline int kvm_need_flush(kvm_ptw_ctx *ctx)
 {
 	return ctx->flag & _KVM_FLUSH_PGTABLE;

arch/loongarch/kvm/mmu.c

@@ -569,7 +569,7 @@ static int kvm_map_page_fast(struct kvm_vcpu *vcpu, unsigned long gpa, bool writ
 	/* Track access to pages marked old */
 	new = kvm_pte_mkyoung(*ptep);
 	if (write && !kvm_pte_dirty(new)) {
-		if (!kvm_pte_write(new)) {
+		if (!kvm_pte_writeable(new)) {
 			ret = -EFAULT;
 			goto out;
 		}
@@ -856,9 +856,9 @@ static int kvm_map_page(struct kvm_vcpu *vcpu, unsigned long gpa, bool write)
 		prot_bits |= _CACHE_SUC;
 
 	if (writeable) {
-		prot_bits |= _PAGE_WRITE;
+		prot_bits = kvm_pte_mkwriteable(prot_bits);
 		if (write)
-			prot_bits |= __WRITEABLE;
+			prot_bits = kvm_pte_mkdirty(prot_bits);
 	}
 
 	/* Disable dirty logging on HugePages */
@@ -904,7 +904,7 @@ static int kvm_map_page(struct kvm_vcpu *vcpu, unsigned long gpa, bool write)
 	kvm_release_faultin_page(kvm, page, false, writeable);
 	spin_unlock(&kvm->mmu_lock);
 
-	if (prot_bits & _PAGE_DIRTY)
+	if (kvm_pte_dirty(prot_bits))
 		mark_page_dirty_in_slot(kvm, memslot, gfn);
 
 out: