[PATCH RFC 2/7] kvm: x86: add a function to exchange value

[Quan Xu <quan.xu0@gmail.com>]

From: Ben Luo <bn0418@gmail.com>

Introduce kvm_xchg_guest_cached to exchange value with guest
page atomically.

Signed-off-by: Yang Zhang <yang.zhang.wz@gmail.com>
Signed-off-by: Quan Xu <quan.xu0@gmail.com>
Signed-off-by: Ben Luo <bn0418@gmail.com>
---
 include/linux/kvm_host.h |    3 +++
 virt/kvm/kvm_main.c      |   42 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+), 0 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 6882538..32949ed 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -688,6 +688,9 @@ int kvm_write_guest_offset_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 			   void *data, int offset, unsigned long len);
 int kvm_gfn_to_hva_cache_init(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 			      gpa_t gpa, unsigned long len);
+unsigned long kvm_xchg_guest_cached(struct kvm *kvm,
+	struct gfn_to_hva_cache *ghc, unsigned long offset,
+	unsigned long new, int size);
 int kvm_clear_guest_page(struct kvm *kvm, gfn_t gfn, int offset, int len);
 int kvm_clear_guest(struct kvm *kvm, gpa_t gpa, unsigned long len);
 struct kvm_memory_slot *gfn_to_memslot(struct kvm *kvm, gfn_t gfn);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9deb5a2..3149e17 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2010,6 +2010,48 @@ int kvm_read_guest_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 }
 EXPORT_SYMBOL_GPL(kvm_read_guest_cached);
 
+unsigned long kvm_xchg_guest_cached(struct kvm *kvm,
+		struct gfn_to_hva_cache *ghc, unsigned long offset,
+		unsigned long new, int size)
+{
+	unsigned long r;
+	void *kva;
+	struct page *page;
+	kvm_pfn_t pfn;
+
+	WARN_ON(offset > ghc->len);
+
+	pfn = gfn_to_pfn_atomic(kvm, ghc->gpa >> PAGE_SHIFT);
+	page = kvm_pfn_to_page(pfn);
+
+	if (is_error_page(page))
+		return -EFAULT;
+
+	kva = kmap_atomic(page) + offset_in_page(ghc->gpa) + offset;
+	switch (size) {
+	case 1:
+		r = xchg((char *)kva, new);
+		break;
+	case 2:
+		r = xchg((short *)kva, new);
+		break;
+	case 4:
+		r = xchg((int *)kva, new);
+		break;
+	case 8:
+		r = xchg((long *)kva, new);
+		break;
+	default:
+		kunmap_atomic(kva);
+		return -EFAULT;
+	}
+
+	kunmap_atomic(kva);
+	mark_page_dirty_in_slot(ghc->memslot, ghc->gpa >> PAGE_SHIFT);
+
+	return r;
+}
+
 int kvm_clear_guest_page(struct kvm *kvm, gfn_t gfn, int offset, int len)
 {
 	const void *zero_page = (const void *) __va(page_to_phys(ZERO_PAGE(0)));
-- 
1.7.1