Re: [PATCH] pci: add enforce_slot_reserved_mask_manual property

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

On Sat, Jan 28, 2023 at 08:20:55AM -0500, Chuck Zmudzinski wrote:
> On 1/28/23 5:26 AM, Michael S. Tsirkin wrote:
> > 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?
> > 
> 
> It's not really needed except for experimental purposes to allow
> an administrator to test experimental configurations with a device
> other than the igd at slot 2. That might be useful in some cases,
> but it is not really necessary unless someone asks for that capability.
> If libvirt users who ordinarily like to manually specify all the
> settings will be OK with the proposed patch to xen that prevents
> an administrator from being able to override a new setting that
> reserves slot 2 for the igd for type "xenfv" machines configured for
> igd passthrough, then there is no need for this patch. I don't think
> many users need the capability to insert a different device in slot 2 for
> the "xenfv" machine type configured with igd-passthru=on, so I would be
> OK if this patch is not included in qemu.
> 
> Chuck

Pls wait and see if that patch gets picked up. Let me know.