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

[Michael S. Tsirkin]

On Thu, Jun 02, 2022 at 08:47:31PM +0000, Lev Kujawski wrote:
> 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>


I don't think this is appropriate in trivial at all.



> ---
> 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.

But pls note I wrote:

        Might need machine compat machinery
        for this.

without said machinery, if guest set one of the other
bits, migration will fail.


>  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
> 
> 
>