xen/pciback: Don't disable PCI_COMMAND on PCI device reset.

BugLink: https://bugs.launchpad.net/bugs/1838700
commit 7681f31ec9cdacab4fd10570be924f2cef6669ba upstream.

There is no need for this at all. Worst it means that if
the guest tries to write to BARs it could lead (on certain
platforms) to PCI SERR errors.

Please note that with af6fc858a35b90e89ea7a7ee58e66628c55c776b
"xen-pciback: limit guest control of command register"
a guest is still allowed to enable those control bits (safely), but
is not allowed to disable them and that therefore a well behaved
frontend which enables things before using them will still
function correctly.

This is done via an write to the configuration register 0x4 which
triggers on the backend side:
command_write
  \- pci_enable_device
     \- pci_enable_device_flags
        \- do_pci_enable_device
           \- pcibios_enable_device
              \-pci_enable_resourcess
                [which enables the PCI_COMMAND_MEMORY|PCI_COMMAND_IO]

However guests (and drivers) which don't do this could cause
problems, including the security issues which XSA-120 sought
to address.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Cc: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/drivers/xen/xen-pciback/pciback_ops.c b/drivers/xen/xen-pciback/pciback_ops.c
index ee2c891b55c6bf46e97e6e529f65e478a12e7c3e..7af047008ea2cd8df35e82363ba73ee2fec49ea9 100644 (file)
--- a/drivers/xen/xen-pciback/pciback_ops.c
+++ b/drivers/xen/xen-pciback/pciback_ops.c
@@ -127,8 +127,6 @@ void xen_pcibk_reset_device(struct pci_dev *dev)
                if (pci_is_enabled(dev))
                        pci_disable_device(dev);
 
-               pci_write_config_word(dev, PCI_COMMAND, 0);
-
                dev->is_busmaster = 0;
        } else {
                pci_read_config_word(dev, PCI_COMMAND, &cmd);