[Xen-devel] [PATCH for Xen 4.13] x86/msi: Don't panic if msix capability is missing

[Chao Gao <chao.gao@xxxxxxxxx>]

Current, Xen isn't aware of device reset (initiated by dom0). Xen may
access the device while device cannot respond to config requests
normally (e.g.  after device reset, device may respond to config
requests with CRS completions to indicate it needs more time to
complete a reset, refer to pci_dev_wait() in linux kernel for more
detail). Here, don't assume msix capability is always visible and
return -EAGAIN to the caller.

Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
---
I didn't find a way to trigger the assertion in normal usages.
It is found by an internal test: echo 1 to /sys/bus/pci/<sbdf>/reset
when the device is being used by a guest. Although the test is a
little insane, it is better to avoid crashing Xen even for this case.
---
 xen/arch/x86/msi.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/msi.c b/xen/arch/x86/msi.c
index 76d4034..e2f3c6c 100644
--- a/xen/arch/x86/msi.c
+++ b/xen/arch/x86/msi.c
@@ -1265,7 +1265,13 @@ int pci_msi_conf_write_intercept(struct pci_dev *pdev, 
unsigned int reg,
         pos = entry ? entry->msi_attrib.pos
                     : pci_find_cap_offset(seg, bus, slot, func,
                                           PCI_CAP_ID_MSIX);
-        ASSERT(pos);
+        if ( unlikely(!pos) )
+        {
+            printk_once(XENLOG_WARNING
+                        "%04x:%02x:%02x.%u MSI-X capability is missing\n",
+                        seg, bus, slot, func);
+            return -EAGAIN;
+        }
 
         if ( reg >= pos && reg < msix_pba_offset_reg(pos) + 4 )
         {
-- 
1.8.3.1


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