[PATCH v2] hw/ide/piix: Ignore writes of hardwired PCI command register bits

[Lev Kujawski]

One method to enable PCI bus mastering for IDE controllers, often used
by x86 firmware, is to write 0x7 to the PCI command register.  Neither
the PIIX3 specification nor actual hardware (a Tyan S1686D system)
permit modification of the Memory Space Enable (MSE) bit, 1, and thus
the command register would be left in an unspecified state without
this patch.

Signed-off-by: Lev Kujawski <lkujaw@member.fsf.org>
---
This revised patch uses QEMU's built-in PCI bit-masking support rather
than attempting to manually filter writes.  Thanks to Philippe Mathieu-
Daude and Michael S. Tsirkin for review and the pointer.

 hw/ide/piix.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/hw/ide/piix.c b/hw/ide/piix.c
index 76ea8fd9f6..bd3f397de8 100644
--- a/hw/ide/piix.c
+++ b/hw/ide/piix.c
@@ -25,6 +25,8 @@
  * References:
  *  [1] 82371FB (PIIX) AND 82371SB (PIIX3) PCI ISA IDE XCELERATOR,
  *      290550-002, Intel Corporation, April 1997.
+ *  [2] 82371AB PCI-TO-ISA / IDE XCELERATOR (PIIX4), 290562-001,
+ *      Intel Corporation, April 1997.
  */
 
 #include "qemu/osdep.h"
@@ -160,6 +162,19 @@ static void pci_piix_ide_realize(PCIDevice *dev, Error 
**errp)
     uint8_t *pci_conf = dev->config;
     int rc;
 
+    /*
+     * Mask all IDE PCI command register bits except for Bus Master
+     * Function Enable (bit 2) and I/O Space Enable (bit 1), as the
+     * remainder are hardwired to 0 [1, p.48] [2, p.89-90].
+     *
+     * NOTE: According to the PIIX3 datasheet [1], the Memory Space
+     * Enable (MSE bit) is hardwired to 1, but this is contradicted by
+     * actual PIIX3 hardware, the datasheet itself (viz., Default
+     * Value: 0000h), and the PIIX4 datasheet [2].
+     */
+    pci_set_word(dev->wmask + PCI_COMMAND,
+                 PCI_COMMAND_MASTER | PCI_COMMAND_IO);
+
     pci_conf[PCI_CLASS_PROG] = 0x80; // legacy ATA mode
 
     bmdma_setup_bar(d);
-- 
2.34.1