vfio/pci: Hide SR-IOV capability

The kernel currently exposes the SR-IOV capability as read-only
through vfio-pci.  This is sufficient to protect the host kernel, but
has the potential to confuse guests without further virtualization.
In particular, OVMF tries to size the VF BARs and comes up with absurd
results, ending with an assert.  There's not much point in adding
virtualization to a read-only capability, so we simply hide it for
now.  If the kernel ever enables SR-IOV virtualization, we should
easily be able to test it through VF BAR sizing or explicit flags.

Testing whether we should parse extended capabilities is also pulled
into the function to keep these assumptions in one place.

Tested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

2 files changed, 40 insertions, 10 deletions

diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
index a171056b41..f2c679e47c 100644
--- a/hw/vfio/pci.c
+++ b/hw/vfio/pci.c
@@ -1772,6 +1772,12 @@ static int vfio_add_ext_cap(VFIOPCIDevice *vdev)
     uint8_t cap_ver;
     uint8_t *config;
 
+    /* Only add extended caps if we have them and the guest can see them */
+    if (!pci_is_express(pdev) || !pci_bus_is_express(pdev->bus) ||
+        !pci_get_long(pdev->config + PCI_CONFIG_SPACE_SIZE)) {
+        return 0;
+    }
+
     /*
      * pcie_add_capability always inserts the new capability at the tail
      * of the chain.  Therefore to end up with a chain that matches the
@@ -1780,6 +1786,25 @@ static int vfio_add_ext_cap(VFIOPCIDevice *vdev)
      */
     config = g_memdup(pdev->config, vdev->config_size);
 
+    /*
+     * Extended capabilities are chained with each pointing to the next, so we
+     * can drop anything other than the head of the chain simply by modifying
+     * the previous next pointer.  For the head of the chain, we can modify the
+     * capability ID to something that cannot match a valid capability.  ID
+     * 0 is reserved for this since absence of capabilities is indicated by
+     * 0 for the ID, version, AND next pointer.  However, pcie_add_capability()
+     * uses ID 0 as reserved for list management and will incorrectly match and
+     * assert if we attempt to pre-load the head of the chain with with this
+     * ID.  Use ID 0xFFFF temporarily since it is also seems to be reserved in
+     * part for identifying absence of capabilities in a root complex register
+     * block.  If the ID still exists after adding capabilities, switch back to
+     * zero.  We'll mark this entire first dword as emulated for this purpose.
+     */
+    pci_set_long(pdev->config + PCI_CONFIG_SPACE_SIZE,
+                 PCI_EXT_CAP(0xFFFF, 0, 0));
+    pci_set_long(pdev->wmask + PCI_CONFIG_SPACE_SIZE, 0);
+    pci_set_long(vdev->emulated_config_bits + PCI_CONFIG_SPACE_SIZE, ~0);
+
     for (next = PCI_CONFIG_SPACE_SIZE; next;
          next = PCI_EXT_CAP_NEXT(pci_get_long(config + next))) {
         header = pci_get_long(config + next);
@@ -1794,12 +1819,23 @@ static int vfio_add_ext_cap(VFIOPCIDevice *vdev)
          */
         size = vfio_ext_cap_max_size(config, next);
 
-        pcie_add_capability(pdev, cap_id, cap_ver, next, size);
-        pci_set_long(pdev->config + next, PCI_EXT_CAP(cap_id, cap_ver, 0));
-
         /* Use emulated next pointer to allow dropping extended caps */
         pci_long_test_and_set_mask(vdev->emulated_config_bits + next,
                                    PCI_EXT_CAP_NEXT_MASK);
+
+        switch (cap_id) {
+        case PCI_EXT_CAP_ID_SRIOV: /* Read-only VF BARs confuse OVMF */
+            trace_vfio_add_ext_cap_dropped(vdev->vbasedev.name, cap_id, next);
+            break;
+        default:
+            pcie_add_capability(pdev, cap_id, cap_ver, next, size);
+        }
+
+    }
+
+    /* Cleanup chain head ID if necessary */
+    if (pci_get_word(pdev->config + PCI_CONFIG_SPACE_SIZE) == 0xFFFF) {
+        pci_set_word(pdev->config + PCI_CONFIG_SPACE_SIZE, 0);
     }
 
     g_free(config);
@@ -1821,13 +1857,6 @@ static int vfio_add_capabilities(VFIOPCIDevice *vdev)
         return ret;
     }
 
-    /* on PCI bus, it doesn't make sense to expose extended capabilities. */
-    if (!pci_is_express(pdev) ||
-        !pci_bus_is_express(pdev->bus) ||
-        !pci_get_long(pdev->config + PCI_CONFIG_SPACE_SIZE)) {
-        return 0;
-    }
-
     return vfio_add_ext_cap(vdev);
 }
 
diff --git a/hw/vfio/trace-events b/hw/vfio/trace-events
index 9da0ff928b..a768fb54ec 100644
--- a/hw/vfio/trace-events
+++ b/hw/vfio/trace-events
@@ -37,6 +37,7 @@ vfio_pci_hot_reset_result(const char *name, const char *result) "%s hot reset: %
 vfio_populate_device_config(const char *name, unsigned long size, unsigned long offset, unsigned long flags) "Device %s config:\n  size: 0x%lx, offset: 0x%lx, flags: 0x%lx"
 vfio_populate_device_get_irq_info_failure(void) "VFIO_DEVICE_GET_IRQ_INFO failure: %m"
 vfio_initfn(const char *name, int group_id) " (%s) group %d"
+vfio_add_ext_cap_dropped(const char *name, uint16_t cap, uint16_t offset) "%s %x@%x"
 vfio_pci_reset(const char *name) " (%s)"
 vfio_pci_reset_flr(const char *name) "%s FLR/VFIO_DEVICE_RESET"
 vfio_pci_reset_pm(const char *name) "%s PCI PM Reset"