[PATCH v8 10/13] vpci/header: reset the command register when adding devices

[Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>]

From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>

Reset the command register when assigning a PCI device to a guest:
according to the PCI spec the PCI_COMMAND register is typically all 0's
after reset, but this might not be true for the guest as it needs
to respect host's settings.
For that reason, do not write 0 to the PCI_COMMAND register directly,
but go through the corresponding emulation layer (cmd_write), which
will take care about the actual bits written.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
---
Since v6:
- use cmd_write directly without introducing emulate_cmd_reg
- update commit message with more description on all 0's in PCI_COMMAND
Since v5:
- updated commit message
Since v1:
 - do not write 0 to the command register, but respect host settings.
---
 xen/drivers/vpci/header.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/xen/drivers/vpci/header.c b/xen/drivers/vpci/header.c
index ae05d242a5..44a9940fb9 100644
--- a/xen/drivers/vpci/header.c
+++ b/xen/drivers/vpci/header.c
@@ -749,6 +749,10 @@ static int cf_check init_bars(struct pci_dev *pdev)
      */
     ASSERT(header->guest_cmd == 0);
 
+    /* Reset the command register for guests. */
+    if ( !is_hwdom )
+        cmd_write(pdev, PCI_COMMAND, 0, header);
+
     /* Setup a handler for the command register. */
     rc = vpci_add_register(pdev->vpci, cmd_read, cmd_write, PCI_COMMAND,
                            2, header);
-- 
2.41.0