[PATCH v1 2/3] xen/vpci: add vpci_read_val helper

[Stewart Hildebrand <stewart.hildebrand@xxxxxxx>]

Introduce a vPCI read helper function that returns a fixed value. This may be
used for RAZ registers, or registers whose value doesn't change. This helper
will be used in a subsequent patch.

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
---
RFC: I'm not a fan of the (uintptr_t) cast, but without it I wasn't sure how to
     pass a value without allocating more memory (either with an xalloc function
     of sorts, or by adding a new element in struct vpci_register and changing
     the interface). Plus, in the current state, to pass a value to
     vpci_add_register(), it may be necessary to cast twice:
     (void *)(uintptr_t)myval. See next patch. Perhaps a new allocation is worth
     it to avoid the casts? Or maybe I'm being overly pedantic and the casts are
     okay?
---
 xen/drivers/vpci/vpci.c | 6 ++++++
 xen/include/xen/vpci.h  | 3 +++
 2 files changed, 9 insertions(+)

diff --git a/xen/drivers/vpci/vpci.c b/xen/drivers/vpci/vpci.c
index acadba1fd6c5..4a96aa50494d 100644
--- a/xen/drivers/vpci/vpci.c
+++ b/xen/drivers/vpci/vpci.c
@@ -133,6 +133,12 @@ static void cf_check vpci_ignored_write(
 {
 }
 
+uint32_t cf_check vpci_read_val(
+    const struct pci_dev *pdev, unsigned int reg, void *data)
+{
+    return (uintptr_t)data;
+}
+
 uint32_t cf_check vpci_hw_read8(
     const struct pci_dev *pdev, unsigned int reg, void *data)
 {
diff --git a/xen/include/xen/vpci.h b/xen/include/xen/vpci.h
index ee7a8fcdee83..17fd252746ec 100644
--- a/xen/include/xen/vpci.h
+++ b/xen/include/xen/vpci.h
@@ -45,6 +45,9 @@ uint32_t vpci_read(pci_sbdf_t sbdf, unsigned int reg, 
unsigned int size);
 void vpci_write(pci_sbdf_t sbdf, unsigned int reg, unsigned int size,
                 uint32_t data);
 
+uint32_t cf_check vpci_read_val(
+    const struct pci_dev *pdev, unsigned int reg, void *data);
+
 /* Passthrough handlers. */
 uint32_t cf_check vpci_hw_read8(
     const struct pci_dev *pdev, unsigned int reg, void *data);
-- 
2.41.0