[Qemu-devel] [PATCH comment tweaked] msix: allow byte and word reading from mmio

[mst@redhat.com]

Although explicitly disallowed by the PCI spec, some guests read a
single byte or word from mmio.  Likely a guest OS bug, but I have an OS
which reads single bytes and it works fine on real hardware.

Signed-off-by: Bernhard Kohl <bernhard.kohl@nsn.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

OK so it could like something like the below. However, my question is:
do we need to put this in or can the guest simply be fixed?

 hw/msix.c |   31 +++++++++++++++++++++++++++----
 1 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/hw/msix.c b/hw/msix.c
index f66d255..38dff59 100644
--- a/hw/msix.c
+++ b/hw/msix.c
@@ -102,10 +102,28 @@ static uint32_t msix_mmio_readl(void *opaque, target_phys_addr_t addr)
     return pci_get_long(page + offset);
 }
 
-static uint32_t msix_mmio_read_unallowed(void *opaque, target_phys_addr_t addr)
+ /* Note:
+  * PCI spec requires that all MSI-X table accesses are either DWORD or QWORD,
+  * size aligned.  Some guests seem to violate this rule for read accesses,
+  * performing single byte reads.  Since it's easy to support this, let's do so.
+  * Also support 16 bit size aligned reads, just in case.
+  */                                                                                                            
+static uint32_t msix_mmio_readw(void *opaque, target_phys_addr_t addr)
 {
-    fprintf(stderr, "MSI-X: only dword read is allowed!\n");
-    return 0;
+    PCIDevice *dev = opaque;
+    unsigned int offset = addr & (MSIX_PAGE_SIZE - 1) & ~0x1;
+    void *page = dev->msix_table_page;
+
+    return pci_get_word(page + offset);
+}
+
+static uint32_t msix_mmio_readb(void *opaque, target_phys_addr_t addr)
+{
+    PCIDevice *dev = opaque;
+    unsigned int offset = addr & (MSIX_PAGE_SIZE - 1);
+    void *page = dev->msix_table_page;
+
+    return pci_get_byte(page + offset);
 }
 
 static uint8_t msix_pending_mask(int vector)
@@ -192,6 +210,11 @@ static void msix_mmio_writel(void *opaque, target_phys_addr_t addr,
     msix_handle_mask_update(dev, vector);
 }
 
+/* PCI spec:
+ * For all accesses to MSI-X Table and MSI-X PBA fields, software must use
+ * aligned full DWORD or aligned full QWORD transactions; otherwise, the result
+ * is undefined.
+ */
 static void msix_mmio_write_unallowed(void *opaque, target_phys_addr_t addr,
                                       uint32_t val)
 {
@@ -203,7 +226,7 @@ static CPUWriteMemoryFunc * const msix_mmio_write[] = {
 };
 
 static CPUReadMemoryFunc * const msix_mmio_read[] = {
-    msix_mmio_read_unallowed, msix_mmio_read_unallowed, msix_mmio_readl
+    msix_mmio_readb, msix_mmio_readw, msix_mmio_readl
 };
 
 /* Should be called from device's map method. */
-- 
1.7.3.2.91.g446ac