[xen staging] xen/vpci: allow unaligned accesses by the hardware domain

[patchbot@xxxxxxx]

commit 79175b13906936855ae354f696f16093e5e29446
Author:     Roger Pau Monne <roger@xxxxxxxxxxxxxx>
AuthorDate: Thu Jul 30 10:07:06 2026 +0200
Commit:     Roger Pau Monne <roger@xxxxxxxxxxxxxx>
CommitDate: Wed Aug 26 11:50:42 2026 +0200

    xen/vpci: allow unaligned accesses by the hardware domain
    
    It's possible for domains to generate unaligned PCI config space accesses
    when using ECAM, and hence vPCI should support those at least for the
    hardware domain.  Such unaligned accesses to the PCI config space have been
    reported to come from ACPI logic.
    
    Relax the checking in vpci_access_allowed() to allow such accesses for the
    hardware domain, and fix the handling in pci_conf_{read,write}{16,32}() to
    fulfill them using MMCFG.
    
    MMCFG regions are identity exposed to the hardware domain, and hence such
    unaligned accesses can only come as a result of the host having MMCFG in the
    first place, as otherwise MMCFG won't be exposed to the hardware domain
    either.
    
    Note that vpci_ecam_{read,write}() already refuse accesses that cross a
    device boundary unconditionally.
    
    Reported-by: Jason Andryuk <jason.andryuk@xxxxxxx>
    Signed-off-by: Roger Pau Monné <roger@xxxxxxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
---
 tools/include/xen-tools/common-macros.h | 2 ++
 xen/arch/x86/x86_64/pci.c               | 8 ++++----
 xen/drivers/vpci/vpci.c                 | 6 ++++--
 3 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/tools/include/xen-tools/common-macros.h 
b/tools/include/xen-tools/common-macros.h
index 88b4a0e5a6..1f9146b23b 100644
--- a/tools/include/xen-tools/common-macros.h
+++ b/tools/include/xen-tools/common-macros.h
@@ -68,6 +68,8 @@
     })
 #endif
 
+#define IS_ALIGNED(val, align) (!((val) & ((align) - 1)))
+
 #define ROUNDUP(x, a) (((x) + (a) - 1) & ~((a) - 1))
 #define ROUNDDOWN(x, a) ((x) & ~((a) - 1))
 
diff --git a/xen/arch/x86/x86_64/pci.c b/xen/arch/x86/x86_64/pci.c
index 8d33429103..6298141c3c 100644
--- a/xen/arch/x86/x86_64/pci.c
+++ b/xen/arch/x86/x86_64/pci.c
@@ -26,7 +26,7 @@ uint8_t pci_conf_read8(pci_sbdf_t sbdf, unsigned int reg)
 
 uint16_t pci_conf_read16(pci_sbdf_t sbdf, unsigned int reg)
 {
-    if ( sbdf.seg || reg > 255 )
+    if ( sbdf.seg || reg > 255 || !IS_ALIGNED(reg, 2) )
     {
         uint32_t value;
 
@@ -39,7 +39,7 @@ uint16_t pci_conf_read16(pci_sbdf_t sbdf, unsigned int reg)
 
 uint32_t pci_conf_read32(pci_sbdf_t sbdf, unsigned int reg)
 {
-    if ( sbdf.seg || reg > 255 )
+    if ( sbdf.seg || reg > 255 || !IS_ALIGNED(reg, 4) )
     {
         uint32_t value;
 
@@ -60,7 +60,7 @@ void pci_conf_write8(pci_sbdf_t sbdf, unsigned int reg, 
uint8_t data)
 
 void pci_conf_write16(pci_sbdf_t sbdf, unsigned int reg, uint16_t data)
 {
-    if ( sbdf.seg || reg > 255 )
+    if ( sbdf.seg || reg > 255 || !IS_ALIGNED(reg, 2) )
         pci_mmcfg_write(sbdf.seg, sbdf.bus, sbdf.devfn, reg, 2, data);
     else
         pci_conf_write(PCI_CONF_ADDRESS(sbdf, reg), reg & 2, 2, data);
@@ -68,7 +68,7 @@ void pci_conf_write16(pci_sbdf_t sbdf, unsigned int reg, 
uint16_t data)
 
 void pci_conf_write32(pci_sbdf_t sbdf, unsigned int reg, uint32_t data)
 {
-    if ( sbdf.seg || reg > 255 )
+    if ( sbdf.seg || reg > 255 || !IS_ALIGNED(reg, 4) )
         pci_mmcfg_write(sbdf.seg, sbdf.bus, sbdf.devfn, reg, 4, data);
     else
         pci_conf_write(PCI_CONF_ADDRESS(sbdf, reg), 0, 4, data);
diff --git a/xen/drivers/vpci/vpci.c b/xen/drivers/vpci/vpci.c
index 0ac9ec8b04..9e2c27e3a3 100644
--- a/xen/drivers/vpci/vpci.c
+++ b/xen/drivers/vpci/vpci.c
@@ -685,6 +685,8 @@ void vpci_write(pci_sbdf_t sbdf, unsigned int reg, unsigned 
int size,
 /* Helper function to check an access size and alignment on vpci space. */
 bool vpci_access_allowed(unsigned int reg, unsigned int len)
 {
+    const struct domain *currd = current->domain;
+
     /* Check access size. */
     if ( len != 1 && len != 2 && len != 4 && len != 8 )
         return false;
@@ -695,8 +697,8 @@ bool vpci_access_allowed(unsigned int reg, unsigned int len)
         return false;
 #endif
 
-    /* Check that access is size aligned. */
-    if ( (reg & (len - 1)) )
+    /* Refuse unaligned accesses for non-hardware domains. */
+    if ( !is_hardware_domain(currd) && !IS_ALIGNED(reg, len) )
         return false;
 
     return true;
--
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