[PATCH v9 11/16] vpci/header: reset the command register when adding devices

[Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>]

From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>

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. Also, honor value of
PCI_COMMAND_VGA_PALETTE value, which is set by firmware.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
---
Since v9:
- Honor PCI_COMMAND_VGA_PALETTE bit
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 | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/xen/drivers/vpci/header.c b/xen/drivers/vpci/header.c
index e351db4620..1d243eeaf9 100644
--- a/xen/drivers/vpci/header.c
+++ b/xen/drivers/vpci/header.c
@@ -762,6 +762,12 @@ static int cf_check init_bars(struct pci_dev *pdev)
         return -EOPNOTSUPP;
     }
 
+    /* Reset the command register for guests. We want to preserve only
+     * PCI_COMMAND_VGA_PALETTE as it is configured by firmware */
+    cmd = pci_conf_read16(pdev->sbdf, PCI_COMMAND);
+    if ( !is_hwdom )
+        cmd_write(pdev, PCI_COMMAND, cmd & PCI_COMMAND_VGA_PALETTE, header);
+
     /* Setup a handler for the command register. */
     if ( is_hwdom )
         rc = vpci_add_register(pdev->vpci, vpci_hw_read16, cmd_write, 
PCI_COMMAND,
@@ -776,7 +782,6 @@ static int cf_check init_bars(struct pci_dev *pdev)
         return 0;
 
     /* Disable memory decoding before sizing. */
-    cmd = pci_conf_read16(pdev->sbdf, PCI_COMMAND);
     if ( cmd & PCI_COMMAND_MEMORY )
         pci_conf_write16(pdev->sbdf, PCI_COMMAND, cmd & ~PCI_COMMAND_MEMORY);
 
-- 
2.41.0