[PATCH 2/2] x86/vpci: refuse to map BARs at position 0

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

A BAR at position 0 is not initialized (not positioned).  While Xen could
attempt to map it into the p2m, marking it as mapped will prevent dom0 to
change the position of the BAR, as the vPCI code has a shortcomming of not
allowing to write to BAR registers while the BAR is mapped on the p2m.

Workaround this limitation by returning false from pci_check_bar() if the
BAR address is 0, thus causing the bar->enabled field to also be set to
false and allowing bar_write() to change the BAR position.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/pci.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/xen/arch/x86/pci.c b/xen/arch/x86/pci.c
index 26bb7f6a3c3a..39fd5a16a4aa 100644
--- a/xen/arch/x86/pci.c
+++ b/xen/arch/x86/pci.c
@@ -101,6 +101,15 @@ int pci_conf_write_intercept(unsigned int seg, unsigned 
int bdf,
 
 bool pci_check_bar(const struct pci_dev *pdev, mfn_t start, mfn_t end)
 {
+    /*
+     * Refuse to map BARs at position 0, those are not initialized.  This might
+     * be required by Linux, that can reposition BARs with memory decoding
+     * enabled.  By returning false here bar->enabled will be set to false, and
+     * bar_write() will work as expected.
+     */
+    if ( mfn_eq(start, _mfn(0)) )
+        return false;
+
     /*
      * Check if BAR is not overlapping with any memory region defined
      * in the memory map.
-- 
2.49.0