xen-pciback: Allow setting PCI_MSIX_FLAGS_MASKALL too

When Xen domain configures MSI-X, the usual approach is to enable MSI-X
together with masking all of them via the config space, then fill the
table and only then clear PCI_MSIX_FLAGS_MASKALL. Allow doing this via
QEMU running in a stub domain.

Previously, when changing PCI_MSIX_FLAGS_MASKALL was not allowed, the
whole write was aborted, preventing change to the PCI_MSIX_FLAGS_ENABLE
bit too.

Note the Xen hypervisor intercepts this write anyway, and may keep the
PCI_MSIX_FLAGS_MASKALL bit set if it wishes to. It will store the
guest-requested state and will apply it eventually.

Signed-off-by: Marek Marczykowski-Górecki <[e-mail]>
Reviewed-by: Jan Beulich <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Juergen Gross <[e-mail]>

drivers/xen/xen-pciback/conf_space_capability.c

···
 };
 
 static struct msi_msix_field_config {
-	u16          enable_bit; /* bit for enabling MSI/MSI-X */
-	unsigned int int_type;   /* interrupt type for exclusiveness check */
+	u16          enable_bit;   /* bit for enabling MSI/MSI-X */
+	u16          allowed_bits; /* bits allowed to be changed */
+	unsigned int int_type;     /* interrupt type for exclusiveness check */
 } msi_field_config = {
 	.enable_bit	= PCI_MSI_FLAGS_ENABLE,
+	.allowed_bits	= PCI_MSI_FLAGS_ENABLE,
 	.int_type	= INTERRUPT_TYPE_MSI,
 }, msix_field_config = {
 	.enable_bit	= PCI_MSIX_FLAGS_ENABLE,
+	.allowed_bits	= PCI_MSIX_FLAGS_ENABLE | PCI_MSIX_FLAGS_MASKALL,
 	.int_type	= INTERRUPT_TYPE_MSIX,
 };
 
···
 		return 0;
 
 	if (!dev_data->allow_interrupt_control ||
-	    (new_value ^ old_value) & ~field_config->enable_bit)
+	    (new_value ^ old_value) & ~field_config->allowed_bits)
 		return PCIBIOS_SET_FAILED;
 
 	if (new_value & field_config->enable_bit) {