hw/display/qxl: Document qxl_phys2virt()

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20221128202741.4945-3-philmd@linaro.org>

hw/display/qxl.h

@@ -147,6 +147,25 @@ OBJECT_DECLARE_SIMPLE_TYPE(PCIQXLDevice, PCI_QXL)
 #define QXL_DEFAULT_REVISION (QXL_REVISION_STABLE_V12 + 1)
 
 /* qxl.c */
+/**
+ * qxl_phys2virt: Get a pointer within a PCI VRAM memory region.
+ *
+ * @qxl: QXL device
+ * @phys: physical offset of buffer within the VRAM
+ * @group_id: memory slot group
+ *
+ * Returns a host pointer to a buffer placed at offset @phys within the
+ * active slot @group_id of the PCI VGA RAM memory region associated with
+ * the @qxl device. If the slot is inactive, or the offset is out
+ * of the memory region, returns NULL.
+ *
+ * Use with care; by the time this function returns, the returned pointer is
+ * not protected by RCU anymore.  If the caller is not within an RCU critical
+ * section and does not hold the iothread lock, it must have other means of
+ * protecting the pointer, such as a reference to the region that includes
+ * the incoming ram_addr_t.
+ *
+ */
 void *qxl_phys2virt(PCIQXLDevice *qxl, QXLPHYSICAL phys, int group_id);
 void qxl_set_guest_bug(PCIQXLDevice *qxl, const char *msg, ...)
     G_GNUC_PRINTF(2, 3);