xen/MSI: don't open-code pass-through of enable bit modifications

Without this the actual XSA-131 fix would cause the enable bit to not
get set anymore (due to the write back getting suppressed there based
on the OR of emu_mask, ro_mask, and res_mask).

Note that the fiddling with the enable bit shouldn't really be done by
qemu, but making this work right (via libxc and the hypervisor) will
require more extensive changes, which can be postponed until after the
security issue got addressed.

This is a preparatory patch for XSA-131.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

diff --git a/hw/xen/xen_pt_config_init.c b/hw/xen/xen_pt_config_init.c
index 68b8f22648aba546cb41e3cb2fc1a4bc58f2efb5..436d0fdc6aba0b80b7cf30b261e9cfbfd34222a5 100644 (file)
--- a/hw/xen/xen_pt_config_init.c
+++ b/hw/xen/xen_pt_config_init.c
@@ -1053,7 +1053,6 @@ static int xen_pt_msgctrl_reg_write(XenPCIPassthroughState *s,
     XenPTMSI *msi = s->msi;
     uint16_t writable_mask = 0;
     uint16_t throughable_mask = 0;
-    uint16_t raw_val;
 
     /* Currently no support for multi-vector */
     if (*val & PCI_MSI_FLAGS_QSIZE) {
@@ -1066,12 +1065,11 @@ static int xen_pt_msgctrl_reg_write(XenPCIPassthroughState *s,
     msi->flags |= cfg_entry->data & ~PCI_MSI_FLAGS_ENABLE;
 
     /* create value for writing to I/O device register */
-    raw_val = *val;
     throughable_mask = ~reg->emu_mask & valid_mask;
     *val = XEN_PT_MERGE_VALUE(*val, dev_value, throughable_mask);
 
     /* update MSI */
-    if (raw_val & PCI_MSI_FLAGS_ENABLE) {
+    if (*val & PCI_MSI_FLAGS_ENABLE) {
         /* setup MSI pirq for the first time */
         if (!msi->initialized) {
             /* Init physical one */
@@ -1099,10 +1097,6 @@ static int xen_pt_msgctrl_reg_write(XenPCIPassthroughState *s,
         xen_pt_msi_disable(s);
     }
 
-    /* pass through MSI_ENABLE bit */
-    *val &= ~PCI_MSI_FLAGS_ENABLE;
-    *val |= raw_val & PCI_MSI_FLAGS_ENABLE;
-
     return 0;
 }
 
@@ -1301,7 +1295,7 @@ static XenPTRegInfo xen_pt_emu_reg_msi[] = {
         .size       = 2,
         .init_val   = 0x0000,
         .ro_mask    = 0xFF8E,
-        .emu_mask   = 0x017F,
+        .emu_mask   = 0x017E,
         .init       = xen_pt_msgctrl_reg_init,
         .u.w.read   = xen_pt_word_reg_read,
         .u.w.write  = xen_pt_msgctrl_reg_write,