KVM: pfncache: include page offset in uhva and use it consistently

Currently the pfncache page offset is sometimes determined using the gpa
and sometimes the khva, whilst the uhva is always page-aligned. After a
subsequent patch is applied the gpa will not always be valid so adjust
the code to include the page offset in the uhva and use it consistently
as the source of truth.

Also, where a page-aligned address is required, use PAGE_ALIGN_DOWN()
for clarity.

No functional change intended.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/r/20240215152916.1158-8-paul@xen.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: Paul Durrant

virt/kvm/pfncache.c

@@ -48,10 +48,10 @@ bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, unsigned long len)
 	if (!gpc->active)
 		return false;
 
-	if (offset_in_page(gpc->gpa) + len > PAGE_SIZE)
+	if (gpc->generation != slots->generation || kvm_is_error_hva(gpc->uhva))
 		return false;
 
-	if (gpc->generation != slots->generation || kvm_is_error_hva(gpc->uhva))
+	if (offset_in_page(gpc->uhva) + len > PAGE_SIZE)
 		return false;
 
 	if (!gpc->valid)
@@ -119,7 +119,7 @@ static inline bool mmu_notifier_retry_cache(struct kvm *kvm, unsigned long mmu_s
 static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
 {
 	/* Note, the new page offset may be different than the old! */
-	void *old_khva = gpc->khva - offset_in_page(gpc->khva);
+	void *old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
 	kvm_pfn_t new_pfn = KVM_PFN_ERR_FAULT;
 	void *new_khva = NULL;
 	unsigned long mmu_seq;
@@ -192,7 +192,7 @@ static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
 
 	gpc->valid = true;
 	gpc->pfn = new_pfn;
-	gpc->khva = new_khva + offset_in_page(gpc->gpa);
+	gpc->khva = new_khva + offset_in_page(gpc->uhva);
 
 	/*
 	 * Put the reference to the _new_ pfn.  The pfn is now tracked by the
@@ -217,6 +217,7 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
 	bool unmap_old = false;
 	unsigned long old_uhva;
 	kvm_pfn_t old_pfn;
+	bool hva_change = false;
 	void *old_khva;
 	int ret;
 
@@ -242,10 +243,10 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
 	}
 
 	old_pfn = gpc->pfn;
-	old_khva = gpc->khva - offset_in_page(gpc->khva);
-	old_uhva = gpc->uhva;
+	old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
+	old_uhva = PAGE_ALIGN_DOWN(gpc->uhva);
 
-	/* If the userspace HVA is invalid, refresh that first */
+	/* Refresh the userspace HVA if necessary */
 	if (gpc->gpa != gpa || gpc->generation != slots->generation ||
 	    kvm_is_error_hva(gpc->uhva)) {
 		gfn_t gfn = gpa_to_gfn(gpa);
@@ -259,13 +260,25 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
 			ret = -EFAULT;
 			goto out;
 		}
+
+		/*
+		 * Even if the GPA and/or the memslot generation changed, the
+		 * HVA may still be the same.
+		 */
+		if (gpc->uhva != old_uhva)
+			hva_change = true;
+	} else {
+		gpc->uhva = old_uhva;
 	}
 
+	/* Note: the offset must be correct before calling hva_to_pfn_retry() */
+	gpc->uhva += page_offset;
+
 	/*
 	 * If the userspace HVA changed or the PFN was already invalid,
 	 * drop the lock and do the HVA to PFN lookup again.
 	 */
-	if (!gpc->valid || old_uhva != gpc->uhva) {
+	if (!gpc->valid || hva_change) {
 		ret = hva_to_pfn_retry(gpc);
 	} else {
 		/*