Re: [PATCH 4/4] hw/ide/piix: Ignore writes of hardwired PCI command register bits

[Michael S. Tsirkin]

On Mon, May 30, 2022 at 03:33:18PM +0200, Philippe Mathieu-Daudé wrote:
> On 28/5/22 22:47, 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>
> > ---
> >   hw/ide/piix.c | 25 +++++++++++++++++++++++++
> >   1 file changed, 25 insertions(+)
> > 
> > diff --git a/hw/ide/piix.c b/hw/ide/piix.c
> > index 76ea8fd9f6..f1d1168ecd 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"
> > @@ -32,6 +34,7 @@
> >   #include "migration/vmstate.h"
> >   #include "qapi/error.h"
> >   #include "qemu/module.h"
> > +#include "qemu/range.h"
> >   #include "sysemu/block-backend.h"
> >   #include "sysemu/blockdev.h"
> >   #include "sysemu/dma.h"
> > @@ -220,6 +223,26 @@ static void pci_piix_ide_exitfn(PCIDevice *dev)
> >       }
> >   }
> > +static void piix_pci_config_write(PCIDevice *d, uint32_t addr,
> > +                                  uint32_t val, int l)
> > +{
> > +    /*
> > +     * 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].
> > +     */
> > +    if (range_covers_byte(addr, l, PCI_COMMAND)) {
> > +        val &= ~(0xfffa << ((PCI_COMMAND - addr) << 3));
> 
> Watch out, len can be 1/2/4.


If there are bits hardwired to 0 the right way to do it is
by clearing a bit in wmask. Might need machine compat machinery
for this.

> > +    }
> > +
> > +    pci_default_write_config(d, addr, val, l);
> > +}
> > +
> >   /* NOTE: for the PIIX3, the IRQs and IOports are hardcoded */
> >   static void piix3_ide_class_init(ObjectClass *klass, void *data)
> >   {
> > @@ -232,6 +255,7 @@ static void piix3_ide_class_init(ObjectClass *klass, 
> > void *data)
> >       k->vendor_id = PCI_VENDOR_ID_INTEL;
> >       k->device_id = PCI_DEVICE_ID_INTEL_82371SB_1;
> >       k->class_id = PCI_CLASS_STORAGE_IDE;
> > +    k->config_write = piix_pci_config_write;
> >       set_bit(DEVICE_CATEGORY_STORAGE, dc->categories);
> >       dc->hotpluggable = false;
> >   }
> > @@ -260,6 +284,7 @@ static void piix4_ide_class_init(ObjectClass *klass, 
> > void *data)
> >       k->vendor_id = PCI_VENDOR_ID_INTEL;
> >       k->device_id = PCI_DEVICE_ID_INTEL_82371AB;
> >       k->class_id = PCI_CLASS_STORAGE_IDE;
> > +    k->config_write = piix_pci_config_write;
> >       set_bit(DEVICE_CATEGORY_STORAGE, dc->categories);
> >       dc->hotpluggable = false;
> >   }