Re: [PATCH] pci: add enforce_slot_reserved_mask_manual property

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jan 27, 2023 at 10:39:28PM -0500, Chuck Zmudzinski wrote:
> On 1/27/2023 8:28 AM, Michael S. Tsirkin wrote:
> > On Sun, Jan 15, 2023 at 07:49:51PM -0500, Chuck Zmudzinski wrote:
> > > The current reserved slot check in do_pci_register_device(), added with
> > > commit 8b8849844fd6
> >
> > add ("subject here") please
> >
> > > ,is done even if the pci device being added is
> > > configured manually for a particular slot. The new property, when set
> > > to false, disables the check when the device is configured to request a
> > > particular slot. This allows an administrator or management tool to
> > > override slot_reserved_mask for a pci device by requesting a particular
> > > slot for the device. The new property is initialized to true which
> > > preserves the existing behavior of slot_reserved_mask by default.
> > > 
> > > Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com>
> >
> > Thanks!
> > I'm trying to think of the best default for this.
> 
> I think it would be better for the default value of
> enforce_slot_reserved_mask_manual to be false, so that a
> user-specified slot will by default override slot_reserved_mask.
> But doing that would change the current behavior of
> slot_reserved_mask.
> 
> Currently, this is the only place where slot_reserved_mask is used in all
> of the Qemu source (code from hw/sparc64/sun4u.c):
> 
> ------ snip -------
>     /* Only in-built Simba APBs can exist on the root bus, slot 0 on busA is
>        reserved (leaving no slots free after on-board devices) however slots
>        0-3 are free on busB */
>     pci_bus->slot_reserved_mask = 0xfffffffc;
>     pci_busA->slot_reserved_mask = 0xfffffff1;
>     pci_busB->slot_reserved_mask = 0xfffffff0;
> ------ snip -------
> 
> I think we could safely change the default value of
> enforce_slot_reserved_mask_manual to false but set
> it to true for the sparc64 sun4u board here to preserve
> the current behavior of the only existing board in Qemu
> that uses slot_reserved_mask.
> 
> What do you think?

I guess first can you answer whether this is still needed
with the latest Xen patches?

-- 
MST