[xen master] vPCI: really no ext-caps without extended config space

[patchbot@xxxxxxx]

commit 873c3db6d2ea6d03a4f61b36d5a285514eac302a
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Feb 2 09:08:17 2026 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Feb 2 10:20:54 2026 +0100

    vPCI: really no ext-caps without extended config space
    
    For DomU-s, whether to emulate accesses to the first 32 bits of extended
    config space as read-as-zero or read-as-all-ones depends on whether a
    device actually has extended config space. If it doesn't, read-as-zero
    isn't correct; not getting this right may confuse functions like Linux
    6.19-rc's pci_ext_cfg_is_aliased().
    
    For Dom0 this then simply allows dropping a later conditional.
    
    Fixes: a845b50c12f3 ("vpci/header: Emulate extended capability list for 
dom0")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/drivers/vpci/header.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/xen/drivers/vpci/header.c b/xen/drivers/vpci/header.c
index aae663bbf5..739a5f610e 100644
--- a/xen/drivers/vpci/header.c
+++ b/xen/drivers/vpci/header.c
@@ -829,6 +829,9 @@ static int vpci_init_ext_capability_list(const struct 
pci_dev *pdev)
 {
     unsigned int pos = PCI_CFG_SPACE_SIZE;
 
+    if ( !pdev->ext_cfg )
+        return 0;
+
     if ( !is_hardware_domain(pdev->domain) )
         /* Extended capabilities read as zero, write ignore for DomU */
         return vpci_add_register(pdev->vpci, vpci_read_val, NULL,
@@ -841,10 +844,9 @@ static int vpci_init_ext_capability_list(const struct 
pci_dev *pdev)
 
         if ( header == 0xffffffffU )
         {
-            if ( pos != PCI_CFG_SPACE_SIZE )
-                printk(XENLOG_WARNING
-                       "%pd %pp: broken extended cap list, offset %#x\n",
-                       pdev->domain, &pdev->sbdf, pos);
+            printk(XENLOG_WARNING
+                   "%pd %pp: broken extended cap list, offset %#x\n",
+                   pdev->domain, &pdev->sbdf, pos);
             return 0;
         }
 
--
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